My 2M4 had similar problems... Turned out to be a bad coil and a bad plug wire.
My recommendation would be to start out with a new fuel filter, check your coil at the local auto parts store (or at home if you've got the specs), and progress to the more elaborate and expensive stuff. I started out by replacing the injector, the ignition module, etc... I finally took it to a mechanic and was told it was the coil... Now I try to think of the most basic stuff first so I'm not shelling out big money for a cheap and simple fix.
Hope you find it.
Kendall (Icelander) Whitlatch
'85 Fiero 2M4 - 5spd Isuzu
'67 LeMans/GTO clone 6.5 litre TH400
All the rest aren't Pontiacs, so what does it matter?