My Fiero wont stay running why?

When I start my Fiero SE(v6) it idles just fine for 6-8 minutes then the RPM drops off and the car almost kills, after which the engine revs way up and slowly comes back down to almost killing. It does this for another 5 or so minutes until it shuts off completely. If I put the car in Gear it dies immediately. It looked like some sort of throttle position sensor was causing the problem, so I jammed a credit card in the mechanical part (to hold the throttle open). This would fix the idle problem for 30sec to a minute but then something else would click and the car would shut off. I am wondering if this is a sensor, computer, or some kind of other issue.