OMG! Someone with the same unknown problem as mine!! I have a 1991 with a new engine, every "tuneup" part possible, battery, alternator, IAC, temp sensor, injectors, coil, O2, and still can not figure this problem out. It's been doing this for the last 3 years with all the new additions never changing the problem. What's worse is that I seem to be the only one that experiences the problem! Not a vacuum line off, runs like a dream cold, lots of pep, until this problem starts. I was so excited 2 weeks ago to finally get an engine code for the first time!!! only for it to tell me it's my fuel system or O2 sensor.... not much help at all.... Throttle position sensor works accurately, idle has been adjusted multiple times to no avail, nothing seems to touch it!! I'm going to clean my EGR and throttle body tomorrow... I'll let ya know what happens

and please... if you have figured it out, send me some ideas!