Last weekend I drove down to Indiana (about 350 miles). A way down the road, I noticed that my revs were unusually high most of the time (about 3500 rpm). The transmission was very reluctant to shift into 4th, and when it finally did, it only did so on a downhill incline (I was using the cruise control). When I got off the highway for some city driving, the car shifted really hard on the 1-2 upshift, enough to chirp the rear tires under very moderate acceleration. It always shifted hard there, but not nearly this bad. And now when it did, there was an audible thump! sound. I stopped and ate with some friends, so the car sat for about 45 minutes. After that, it worked like it always did. When I drove it home on Sunday, I went about half of the trip on the highway without it ever shifting into 4th. I pulled off the highway for a pit stop, and when I got going again, I realized the car wasn't going in 1st or 2nd either. It just started off in 3rd. I checked my ATF yesterday and found it to be a little low, so today I added some and drove around a little bit to see what it did. No difference, just always in 3rd. I even put the shift lever into 1, 2, and D (no square), and it made zero difference. HELP!

car is a 1996 Z28, I bought this trans this spring with 3k on a rebuild.

 

Have you read this yet? https://camaroforums.com/forum/93-02-transmission-differential-122/4l60e-common-problems-51998/

 

just skimmed it, doesn't really seem to match my problems though.