My bet is on the injectors.
If they haven´t been replaced/cleaned previously.
Easy check is to measure resistance on them.
Just remove the clip on them, put the leads of a voltmeter (ohm scale) on the injectors contacts.
Cold, they should all read around 16 ohms, warm a bit higher (?).
Thing is, they should all read about the same.
I would bet you have one or two reading really low, like 5-9.
Are they grey colored, they would be the original ones, GM put them in the 89-90-91 models with a (then) better design.
Turned out they weren´t.
Almost everyone with those MULTEC´s have replaced them with better ones.
If it´s your problem of course.