My Fiero has some crazy idle problems

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.

What year?

Check for codes?

check for vac leaks?

Famous for broken exhaust manifold bolts...

You could possibly take out and check the EGR valve for carbon build up and it also could be your IAC (idle air control) for problems if those are not the issues than it could be what Tech II said