Good suggestions. While thinking it over last night, I reasoned that if it were an ignition issue I should be able to duplicate the scenario with the engine running in park and revved to the same rpm range. If it doesn't do it in park, then I know it is not ignition related.

I think you might have hit the nail on the head that the PCM makes these changes for smoother shifts. On the 1-2 shift, the car bogs down for about a half second, then when it does shift, it shifts hard enough to push you against the seat, especially on medium to hard acceleration. To me, it feels like the shift kits we used to put in our cars, where you replaced the accumulator spring with a stiffer one to give you firmer shifts.

I am going to drive it again and see if I can see the spark adv change happening on the 2-3 shift also. The 3-4 shift is very smooth, but there is a slight bog or delay on the 2-3, but barely noticeable.

Forgot to ask, is there a separate transmission control module? The parts stores show one, but everything I have read so far seems to point to the PCM for controlling everything and I cannot find anything that looks like the pictures of a TCM that I have seen while browsing auto parts sites like Rock Auto or Oreilly'*. I know where the PCM and BCM are.
Also, just like the underhood fuse block, the PCM has been exposed, uncovered, to the elements for who knows how long. I wonder if I should unplug the three harnesses and check for corrosion.

I am becoming more and more convinced it is the PCM. I did unplug the three connectors and sprayed them with electrical cleaner and blew them dry. There was some dirt on the inside of the plugs but all the terminals looked good. I also tested the TPS with the engine off. I checked TPS voltage, TPS resistance, and also watched in Graph mode. Nice smooth reading. I test drove it again and checked every engine related sensor - TPS, VSS, MAF, MAP, EVAP, Fuel input, anything I could find that was engine performance related. I ran the graph alongside the spark advance graph to compare, and they all had nice smooth curves even when the spark advance dropped off. I also have a MItsubishi Galant I have fixed up to resell, so I drove it; under hard acceleration you can just barely make out the drop in power before it shifts, and only because I was waiting for it I suppose.

It seems that the PCM is just slow to respond? As I said there is a much slighter lag in 2-3 and virtually none in 3-4. It stays in gear, downshifts well and doesn't 'hunt' for gears while cruising or accelerating so I don't think it is a transmission or shift solenoid issue. No codes have come back during the last three test drives.

So my next question is should I drop the cash for a new PCM? Does a new one have to be programmed? Would a used one be worth trying. I remember from years ago while attending classes at a GM Training Center, they always said 'replace with a good known part.' If I had another MAXX I would swap out the PCM but I don't have that advantage.

I found used PCMs on eBay for $30-35; I compared the OEM number and service number to mine which are identical. The only difference is the four digit code, mine is 5313 vs their 5318 and the serial number under the bar code, but I would imagine these are simply production numbers. The eBay ad says they are tested and come with lifetime warranty but do have to be programmed to the vehicle. I am going to check around and see if anyone in this small town does that. $35 is a small gamble.

It knows when it is in park or not. It also measures how much torque is being supplied, throttle position, etc. then sets spark advance and fuel supply differently based on this. In short, it will do everything differently when free-revving. Pre-computer engines did a lot less of this (but still some) in the same conditions.

I believe it does not.

I'd love to see a picture of it.

It'* designed to be exposed, but not necessarily with the hood open and definitely not with the wiring disconnected.

I suggest you have someone scan this car with something that can activate solenoids individually as well as read pressures and sensors and etc. such that you know what'* really going on in there.

Haven't had time to work on the car for about a week but I did find out that the closest dealer that can program a used PCM wants $130.00 to do it and they are 75 miles away. I can buy a reman unit on ebay and have it programmed for about $150.00 so it would be cheaper to go that route. I am going to see if I can find someone who has a scan tool that can read and monitor the transmission and won't charge me the value of the car to hook it up and check it out.

If you drive it 'granny style' you barely feel the 1-2 and 2-3 lag. The harder you accelerate from a dead stop. the more severe the lag is. The TPS reads on a smooth curve, I watched in with the vehicle sitting and while driving. All other sensors show smooth curves also while driving as I mentioned above.

Will let you know what I found out.

Looking forward to it.

Regarding your shifting issues it sounds like weak springs in your tranny'* accumulator. If you have the 4T65E tranny those are notorious for lazy 1-2 and 2-3 shifts. Once the PCM detects abnormally long shift times it compensates by dialing up the line pressure to the max. I had these issues with my Lesabre and the TransGo shift kit solved it. It'* shifting quickly and smooth almost as if the tranny had been rebuilt. So for about 40 bucks for the kit and 2 gallons of fresh fluid and a filter life is good again with my car. Just my two cents.