yours is an 05 right?
Yes mine is an '05 GTP. Just traded in the '02, and also had an '00, neither of which had tranny problems, but may have just been the roll of the dice.
i tried a 97-98 and 99-by total chance
best one was from jjs -
it didn’t fail for me at all fully modded with a shift kit
i got lucky i guess
Here is a page with some info on the 4T65E tranny. Not sure if it has your exact problem listed but might provide some help.
I have a '00 GTP with 86k on it and I am just starting to have some problems with the torque converter unlocking and locking at low RPMs under load. Though I knew when I bought it 5 years ago there would eventually be a problem with the tranny I would have to deal with.
psssst… you forgot the page 
Depends on the dealer. At cochran we have 3 tranny guys. One does the trucks and the other 2 do the cars. Both of them have been doing it for years and rarely have come backs.Im not a mechanic but just from working around in the shop a shutter at low RPM in between gears sounds like a torque converter.just my $.02.
also make sure you make a note that the tranny has to be hot or like you said request to ride with a tech.
Yeah, I never trust that they actually put the stuff in the computer correctly. Last time I printed out a detailed description of all the problems I was bringing it in for and handed them the paper. 
LOL…man now I feel really dumb! Ok here it is this time.
http://www.tripleedgeperformance.com/4T65E_Transmission_Info.php
your problem might just be the TCC
That is what I am hoping to. I have been trying to do alot of research to have a real good idea before I dig into fixing it.
define fully modded. too many people have or had gtps that i never heard of
pulley, gutted cat -3 in dp
hybridynamics fenderwell cold air intake
mechanical shift kit in the tranny- stuff
this way before your time when club gp was in full force
we dyno dayed/ raced/ modded once a month
99-2004- rip
cool. it definitely seems i missed the boat here, oh well fuck it. I’m wanna throw some shit in the spare tranny i have now. I wanna put 3.29s in it, new input shaft and get a 3000 stall. I can’t believe my GT tranny has taken everything i threw at it. 1st gear is murder
That’s heavily modded ???
gonna say its a valve in the trans
my blazer did the same thing and i had to change the valve body in the trans… 60 dollar fix
Took it back in again and they duplicated the problem this time. Did a rebuild and replaced 2nd gear clutch pack & valve solenoid. Also had an oil pan leak. Total cost of parts and labor was $1800. But I still had 3500 miles left on the extended warranty. 
congrats
solenoid. I believe there are actually 2 of them that tend to go bad. never seem them start acting up at 58k but at least its fixed under warranty.:bigthumb:
ya the Pressure Control Solenoid sets a DTC, that is “max adapt long shift”, very common in the 4T65’s
i have seen the code appear anywhere from 20K to 120K
Now its time to start Modding like us other 3800 freaks
I don’t believe mine set any codes. For what it’s worth, the solenoid was part #24225825. And the 2nd gear clutch TSB was 06-07-30-23B.
As far as modding, it’s staying completely stock. College starts in under 2 years and I need this thing to last! One broken toy is enough for me. 