This is going to be a long post, so get your coffee ready I have a 1996 Riviera 3800 supercharged. It was idling really low at times, usually after I had been driving it for a while, letting it sit for a bit, and went to start it again. Sometimes it would even stall. I did some research and decided to clean the IAC motor. Well, when I did this, I accidentally took off the TPS sensor. I put it back on - first wrong, and it was idling around 2500 RPM. I put it back on - maybe the wrong way still - I don't know. I broke the IAC motor so I went and bought a new one of those and put it on. Anyways - here is my problem now - that I have never had before messing with all of this: When I start the car, everything is normal. It idles perfect, runs like a champ. When I put the car in drive, everything seems normal until I am either hitting a certain RPM or MPH, and I can feel the car shift into another gear - 2nd or third. And it just stays there. There car doesn't shift. When I come to a stop - its like trying to take off in a stick shift in 3rd gear.

I unhooked the battery cables from the battery and touched them together to reset the computer after cleaning the throttle body where the IAC motor goes in and put the new IAC motor in.

I don't know what to do next. I don't know if I still don't have the TPS sensor in correctly or what. Tonight when I get home from work I was going to try unplugging the TPS sensor and driving it and seeing what happens.

Any thoughts?

check the tps sensor connector make sure there are no loose pins or pinched wires

Will do. I'll reply this evening to let you know what happens.

Been meaning to do this for awhile. Installing a TPS sensor can't really be explained on paper, and video is so much better. So, here'* a quick video I shot installing the TPS sensor.

Excellent TPS video Mike!
Alignment of tabs required ~!!

So the first time I put it back on I had it on the wrong side of the post. The second time I figured I had it on wrong and I'm pretty sure I put it on the underside the second time, but sitting at work here I guess I'm not sure. When I get home I'll take it off and put it back on, check the connector and the wires and let you know what happens. Thanks for the video - there were NONE on YouTube showing how to do that. Like you said, it'* pretty hard to explain

Well checked everything on the tips sensor, took it off and made sure it was on right. No change :( Still acts like its stuck in a high gear. Any other thoughts?

TPS sensor. Stupid autocorrect

Electrical connections all look good.

With the Rivs, its not uncommon to have symptoms like you explain due to overcharging. A bad alternator is the most common case I have read about. I would have the alternator tested and also measure battery voltage with the car running. Should be between 13.5-14.1V I believe? If not a member already I would suggest joining and posting about your issue here as well:
Riviera Performance - Dashboard

Best of luck
Dave