It really sounds like you have a bad pressure control solenoid. If you scan the computer, you may find a P1811.

Any difficulty in replacing that? Can I just lower the engine in the subframe and pull the side pan off? Can it potentially damage anything if I don't replace it immediately?

well, the PCS does what the vacuum modulator used to, which is regulate pressure, specifically during shifts. if there'* not enough pressure or shifts take too long, it will burn the clutches to nothingness pretty quickly near WOT with enough torque...

Replacing the solenoid is a very straight forward task. You unplug it, pull a small clip, and then wiggle the solenoid out. Reverse to install new one. Getting to it is a bit more involved. Check your sub frame, mine has a split frame. I can unbolt the frame just behind the drivers side front wheel where the sway bar is, and at the front passenger side corner. You pull both axles, support the engine from above with a 4x4 and a chain. Drop the sub frame if it splits. If not you need to unbolt the steering rack and wire it up. Then use a jack to lower the engine/trans. Oh ya, drain trans fluid before pulling the axles. Get it just low enough to remove the side pan. Locate the solenoid and change it.

Sounds just like the last job I did with the parking pawl rod replacement.

The weird thing is, it shifts perfectly, in all gears, it just occasionally does this stupid thing.

I guess if the consensus is that it'* bad, it will be the first thing I do when I get this thing back from California.

Also, Robert, when it shifts weird, it shifts really quick, as if it had a shift kit. It reminds me of the time I cut a few wires to try to wire up a switch to manually control the torque converter clutch and every shift would engage almost instantly.

I'm going to try to some seafoam trans tune and see if that helps anything, since the transmission probably sat on a shelf with 6 quarts of fluid for who knows how long before it got sold. Bob thinks something might be clogged up in there. Its weird.

hmm.... anything in the tune to modify shift times or pressure based on either coolant or trans temp? IIRC, even the 94-95 1500 truck PCMs had that option...

Really not sure what Bob did with the transmission tune. I think he'* going to have another look at it when I get that fuel pump in and get the trans shifts dialed in more properly. We'll see.

Scan it for codes. P1811 will give you a full pressure shift, it'* the max adapt code. Aka limp mode.

Looks like you guys covered all the bases! I'm in PA for a week on vacation.

Sounds exactly like what its doing. At least a full pressure shift is better than a no pressure shift, lol! If it would even shift!

Anyways, this is a 2003 transmission, so hopefully someone can shed some light on this animal. He used the tune from a 2004 Impala thinking that the internals would be the same. Was this the right move, or should he have been using the tune from a 2002 w-body? Or do I need to find someone with a 2003 w-body to tune it?

Today it didn't shift right at all going to work. Very quick and sometimes harsh engagements. Actually, going from drive to neutral would give you a good jolt, if you were moving. Shifting from park to reverse to neutral to drive and all in between acted fine while stopped, but still more noticeable than it should be.

Wednesday I'm getting a new fuel pump in there to deal with fueling issues, at which point we'll be flashing a new tune and will be able to scan for transmission codes.

Any other help or insight here would be appreciated.