I've had champion plugs start going bad and do the same thing. Spent $400 on a head job (in 1982) because of it. I'd start there, 'cause it's cheap. If that doesn't do it, you might pull your O2 sensor (it does have one, right?) and heat it up with a torch and make sure it's working. Other than that, I simply don't have a clue.
81 Cherokee, 360 auto
88 GW, 360 auto, perfect body and now running.
88 Comanche, 6 banger, auto w/overdrive (I'd really like to get rid of this stupid thing...I can't stand it)
(UPDATE: Chanting "I got my glaaaassss, I got my glaaaasss")