Background info: I have a'99 Pontiac Grand Prix GTP with the 4T65E-HD transmission and 124,000 miles on it. My dad and I recently changed the pan gasket and drained the fluid as involved in that process. I noticed the issue about 6-8 months ago, and changing the gasket fixed a leak, but not this issue. The fluid was getting a little dark, but was about on schedule for a change. It does not smell burnt on the dipstick. There was a moderate amount of normal sludge on the magnet, but no metal shavings/hard pieces. It felt like grease. I also had a transmission shop look at it for a free inspection in August before driving 1000 miles to school, and they said things seemed fine, but my transmission computer wouldn't communicate with their computer, and they said the shift I'm concerned about was a little soft.
The Main Symptoms: The shifts are fairly smooth usually, as far as I can tell. (I haven't driven many different cars to compare because I'm only 21 and a girl.) When I have the car in "D" I can watch it shift through 1-2-3 with light throttle pressure by watching the rpm gauge(and listening to the loud exhaust). However, when it shifts again, into overdrive, the rpm gauge jumps up maybe 200-300 or so before it settles down to the 1700 rpms that it stays at while in overdrive with the TCC locked up (I always forget if the TCC locks or unlocks at this point, but whichever one it's supposed to, that's what it does here.) It sounds a little bit like when you forget to let off the gas all the way when pushing in the clutch on a standard. If I accelerate more quickly, which I don't do often, but my dad has tested it out, I'm not sure if it goes into third gear. It appears that it may just skip third and go from 2nd to overdrive in this case.
At first, one might think this is the common problem of the 3/4 clutch or the 4th shaft going out on this car, but I don't think that's the case. When I start out with the shift selector in "3" instead of "D," It will shift just fine 1-2-3. Then, if I wait until it's past 47 mph or so to slide it into D, it shifts normal, just like the other gears. I can also prevent the problem if I have it in drive and let off the gas at just the right moment as it shifts. And, one time, I started in third and slipped it into drive just a little too early, like at 44 or 45, and it seemed to shift down right away, but then I heard the rpms jump a small amount very briefly. That made me think that the TCC might be making it do that, but I'm not sure.
A Little More:
I went home to the Midwest (I go to school in the South) over Christmas and noticed more. . . When the engine is cold and I start it in "3," or "D," for the first five miles or so, it either doesn't shift into overdrive or the TCC doesn't lock up (unlock?). The rpms are lower in D than they are in 3 though, so I think that was just the TCC, which I've heard doesn't work until the engine heats up anyway. However, in this situation, I did notice that whenever the rmps went down the the normal 1700 after the car heated up (like 2200 until then in D, more in 3), they didn't jump like they would if the car had been warmed up.
Also, my shifter for PNRD321 seems to be a little loose. I think it might just be the year because before like 2002, they made it so you could easily go between 3 and D without pushing in the button (I think), but I thought I would include the information in case it might be affecting the issue.
Thanks for any ideas anyone might have!