stock 01 XJ. if I start it up and run the A/C, I get nice cold air. After a period of time (usually at op temp) I just get warm air out of the vents. this will last from 30 sec to an indefinite duration of time. Usually, after an unknown period of time, I get cold air again. If I happen to have to stop and shut the engine off, I get cold air again after the shutdown period. My A/C mechanic had it for two days and could not find anything wrong.My thoughts are either something going bad or heat effecting something. looking for input. The Mrs ain't happy.

 

Is your electric fan coming on?If not it can cause the same symptoms. Check your high psi switch,the one right off the compressor.2000/2001 use this switch to tell the pcm to turn the fan on when psi and temp get high. Very common failure. I had the same problem.