Exactly my symptoms with the '91 I bought last year with 17,000 miles.
Ran fine on short test drive but the first time I got it good and warmed up - sounded like the flywheel was going to come through the hood.
Marc's article describing the injector coil failure and testing procedures from the ECM connectors helped me confirm two bad primary injectors.
Check it out:
http://www.zr1specialist.com/HAT%20W...in%20Place.pdf
Recommend this vendor for replacement injectors:
http://www.corvetteinjectors.com/sho...p?productid=32