My 1990 Audi 90 quattro seems to run fine with good acceleration and performance until it worms up and runs for about 20 minutes or so. After that it feels like it's just not getting power as it should. My guess is it's just a tired motor but It could also maybe be the injectors, the intake, the valves, etc. Does anyone have any ideas what the problem could be, the car has 152 thousand miles on it and the top half of the engine was rebuilt around 230 thousand.

 

So your planning on rebuilding thetop half rebuilt at 230K but have152 now!
Scheduled maintenance taken to extremes!!!

 

I meant 252 thousand miles on the engin now...

 

Before you do anything have a compression and leakdown test performed. This will tell you quite a bit about the health or lack thereof of your engine. Then you can make an informed decision rather than guessing.

 

The compression is 200 across the board, as for a leakdown test, I have no idea what that is...

 

200 is excellent compression. Leakdown measures how quickly that compression declines over time. If you have worn rings or valve seats, the compressed air in the combustion chamber will leak out. If the air leaks out the exhaust then you have valve and valve seat wear. If it leaks out of the crank case then the rings and/or cylinder bores are worn. You will definitely hear the air leaking out. With such high compression my guess is that you have no problem at all with the basic engine. I sincerely doubt it is "tired", not with 200lbs of compression.
Your driveability problem is electrical. My initial suspicion is a weak coil.

 

Could it be a bad o2 sensor?

 

A bad O2 sensor might be a problem. Are your cat and O2 sensor the original parts? Do you have your car checked for emissions regularly where you live? In California I have to do this every two years to keep my car registered. I know my cat is going because over the last three tests the NOX ( oxides of nitrogen ) level has risen almost to the point of failing the test. A five gas analyzer can tell you a lot about what is going on in your engine. I believe when an O2 sensor goes bad you will see a check engine light, but I'm not sure how bad the O2 sensor has to be for this to happen. Have you tried to retrieve any blink codes that might be stored in the car's memory? This is another tool a lot of shade tree mechanics fail to use. Often times the car will tell you what is wrong with it through the blink codes.