Bought my 90 vette one year ago.  Been running progressively worse over time.  Hard to start,  rough idle then it will eventually clear.   But if you take it out and hit anything above half throttle, skip - miss- bog - shutter.  Once I lift it clears up.  I took it to Zip Corvette in Mechanicsville VA they found nothing wrong with the computer check and suggested the MSD box was defective. I removed the MSD and since replaced:

Distributor cap and rotor,  coil, plugs, wires.  Still there.   Any ideas?    

My 88 runs good until 2700 rpm in any gear then stumbles,if I let off gas a- ok again  

have you try looking for any vacume leaks?  i have a 91 that ran rought and found lots of cracked vacume hoses. after i replaced them car ran alot smoother.hope it helps you