Ive been having 3 consistent issues with my 91 Z with the 5.0 TPI. Engines been rebuilt.
1st issue: when I have the throttle only open a little and its cruising at lower RPMS (example, 45 MPH in Overdrive at around 1200 RPMS), you can feel random misses, doesnt make noise and it only does it when the throttle is lightly opened at a cruising speed.

2nd issue: this is another tricky one, when the car is barely warmed up (temp gauge is a little over the 100 mark after a short drive) and you start it up again and try to drive, it has lean misfires real bad, and goes away after a few seconds of driving or if you stomp on the gas a little. Does the same thing when its at about 220. Doesnt do it while driving at all, or starting at any other temps.

3rd issue: yesterday, I noticed my my tach is starting to jump a little bit at random. I replaced the chip behind the tach already (just redlined before I replaced it)

and forgot to mention, for some reason, I jumped the OBD port and pulled a code 43. doesn't display a SES light at all for it though. (caused a code to make sure the light still worked...which it does)

Cars got a bunch of new parts including a whole new set of injectors, so I can think of the 22 year old catalytic converter, egr solenoid, or a faulty distributor causing the problems? I am really lost at this point, Ive thrown to many parts to keep guessing blindly..

 

Diagnose the reason for the code ; ESC Spark Control

http://www.chevythunder.com/esc_map_code_43.htm