Glad you guys found the problem. Yes the early cars 90-91 seem to be the most susceptable to lack of use where the injectors are concerned. Remember that if you go to OEM?GM injectors that GM never "corrected" the seals for ethanol.
I think the Delphi injectors are corrected. I also know that a bunch of people have used the Accel injectors with very good results. There are threads about which P/N Accel's to use here. And there are always the RCEngineering.com injectors we get a registry discount too from them.
JMHO on this job if I may. Don't piece this job because they will all fail the same way. Just bite the bullet and do them all(16) at once so you only have to do it once. It's a lot of cash but in the long run it's just better to be done with it.
You also might want to consider giving the secondary port throttle system a very critical "eye ball" while you're under there. Ya know the control solenoid, tank, vacuum line spyder, secondary actuators, check the CCVBox cover for loose bolts & leaking gasket & hoses, clean the valley & starter, load test the alternator, plug wires and coils, A/C system vacuum control valve, temp sensors(in the left I/H, up forward), serp belt, make sure the screws on the secondary butterflys are tight even a drop of lock tite on them is cool(one guy had one fall off & go into a combustion chamber). Or maybe even a port & polish on the plenum, I/H's, T/body with some new powder coat!
Sorry for spending your $. Point is the gaskets are not cheap and GM don't have 1/2 of them anyway. GM don't sell them in a set either, just all seperate. Try partszoneonline.com (P & G chevy in PA) or Superrior Chevy or Marc Haibeck (you could ask him the best source for the gaskets).
Okay I'll shut up now!
Tom