Well . . . I don't want to feel TOO good about this yet, but the problem seems to be solved. Installed a new mass airflow sensor, test drove with the scanner for well over an hour and had no issues.
The one measurement we were looking at was in relation to the TPS. Hovered between 4 and 6 before with no throttle, down to 1 or 2 on stop and perhaps more importantly scaling down to 1 or 2 before the vehicle stopped. It was remarkable that you couldn't even feel the downshift to 1st.

What threw them upon picking up the MAF was that it was just the sensor when they indicated buying it previously it included the surrounding housing. A little struggle to depress the clip holding the sensor in the housing but eventually got it out.

Anyway, we shall see. But again, I really do want to express my appreciation to CathedralCub and carfixer for your input and help.

Thanks for the update! I'm surprised to hear that this stabilized it. I'm curious to see how this turns out in the long run.

Thanks CC. Me too. So far, three days later and no problems. The first long drive for me will be telling, but we drove for well over an hour this past Monday and no issues.

I'm quoting this from an online source but the transmission folks found something similar in their printed materials. Since the PCM uses air flow readings to determine the shift points of the automatic transmission, if the air flow sensor doesn't work properly, it could alter the shift pattern of the transmission.

The main mystery still to me is after replacing the PCM, then the throttle body, what explanation is there for the car going 500 miles and no issues then returning? Regardless, I think we're good for now.

It might have been taking a while to re-learn all of the new parts.

Well . . . the problem has returned.

About 29 F in the mornings where I live, I took my daughter to school today and noticed the problem. The problem that occurred when warm is now happening when cold. A thought that occurred to me was I hadn't changed the air filter in about 10k miles, even though I could see light through it - maybe restricting enough air flow to be causing an issue with the mass air flow sensor which seemed to fix the problem. Bought an STP high quality filter this morning and it seemed to fix the issue. It even seemed to give better response from pedal. Yay! Drove through the day, to work, out to lunch, then home - no problem. Ran to a local home improvement store a short while ago, parked, got back in the car 20 minutes later and the second shift starts returned. I even manually shifted down to force it into 1st and started up in 2nd gear through two stop lights.

Back to the transmission place on Monday.

How long is a reasonable period for a re-learn?

Darn!

I wouldn't suspect the air filter if you can see light through it.

I agree, it needs to go back to the transmission shop. They should have the ability to read what is commanded, what is sensed, and what actually happens during each attempt and use that to diagnose.

I can't remember this one: Has anyone done a transmission filter change?

A few start/drive/warm-up/drive/shut off cycles at least.

Darn indeed and thanks yet again for the reply CC. The transmission filter was changed in the first go-round when they replaced solenoids, sensors, installed the shift kit, etc. I've already put the transmission shop on alert I'll be seeing them Monday morning.

I also found a couple of other resources online I've been investigating.

• Drivetrain Resource - Owner of the site is a guy named Jason who seems pretty well networked. I emailed with my issues and maybe he has some resources or references.
• Sonnax - I bought one of their valve bodies. Wouldn't hurt to possibly engage them for some help since they are transmission experts after all.
• ATRA (Automatic Transmission Rebuilders Association) and ATSG (Automatic Transmission Service Grou
• BobIsTheOilGuy Forum - Seems to get a lot of activity similar to here with people knowledgeable in things other than oil.

This is driving me nuts but I will get to the bottom of it.

An update for the group.

I read several references to Dave, a tech at Triple Edge Performance and a primary source of knowledge specifically on the 4T65E. I emailed my issues and we engaged in a back and forth of a few emails. Great guy for sure! At this stage, he is in agreement with my transmission shop that the problem is either in the PCM or outside the transmission and likely the wiring. As long as the PCM is commanding the correct gear, it is safe to say the transmission is doing its job. He went on further to say he'* seen several occasions where there is a damaged wire inside the main wire harness under the air filter housing, either from a poor crimp on a terminal, wire break inside the insulation, etc. They don't throw a code but give the appearance the solenoid is at fault. This was in actuality the next step for us and Dave just reinforced what we thought.

Someone else made the recommendation to swap the new throttle body purchased a few months ago with the old one, which we did. We then also pulled the wire harness connector to the transaxle case, cleaned the connector on the case and the harness, applied some dilectic grease and reinserted. Did the same actually with the connector and cable leading to the throttle body. Surprisingly, although I haven't made any long drives, I haven't had any issues with the downshift to 1st. Also, haven't been out on the freeway to test between 3rd and D.

The transmission shop has a relationship with a connection to ATSG and indicated they would reach out to discuss. I told him I would attack things from the Buick side.

Called Buick mainly looking for TSBs and any kind of history of customers with this same issue. The rep indicated I needed to have an authorized GM servicer perform a diagnostic before they could do anything. When I tried explaining to the rep that with over $3,000 in transmission repairs, even the purchase of aftermarket parts would surely have provided enough royalty back to GM to more than cover a few times over the cost of a diagnostic. She then recommended going to my local authorized servicer and see if they would help. When I proposed getting the diagnostic done, then what could they do? She said they could make some "concessions". A 17-year old car with no remaining warranty on ANY part of the vehicle, what concessions ?!!?? You gonna give me 2% discount on a brand new GM vehicle . Haha.

Well, I went down to the local GM dealer, talked with the parts manager who has access to TSBs and was able to get a few, took them back to the transmission shop for their thoughts. If I get the diagnostic done they can run checks to see if the PCM is updated to the latest firmware update, but there would obviously be a separate charge for the re-programming.

For the time, I'm good but at this point will look to target new harnesses. An AC Delco harness connecting the transaxle case and several components runs $230, not including installation. Since that is where the problem seems to be isolated, for the peace of mind and to rule out one less item, probably a good investment. I'll report back if we glean anything further on this.

Thanks again to everyone for the assistance and support.