Could be a bad fuel injector, or much worse- a burnt valve. If it's not
ignition-related, it's either fuel or mechanical. You'll need to narrow it
down to one cylinder if you can. If not one offending cylinder, could be a
bad O2 sensor. Others here will probably offer more accurate diagnoses, but
that's where I would start.
Good luck.

Chris
90 & 94 GXE's