94 Sunbird keeps stalling

I just bought a 94 pontiac sunbrd 3.1 6cyl automatic at around 104,000 miles. When I took it for a test drive it ran perfect, as when I drove it home. I've been driving it regularly for about 2 days now, and whenever the engine drops below 1,500-2,000 rpm it stalls. I've changed the oil and spark plugs, and the air filter looks fine. The guy I bought it from sat for a few months with an almost full gas tank. I asked the guy at the auto parts store and he said either the gas in the tank is bad, or I need to replace the throttle position sensor thing. So I bought the sensor figuring if I run through the gas and it still stalls, I'll switch out the sensor. But if it turns out it's the gas, I'll just return the throttle sensor.

I should also add that, before it stalls, the idle speed will shoot up, then drop back down to the point of stalling. Then when I turn it back on, I have to give it a good amount of gas to keep it from stalling again. I'm really getting sick of having to put it in nuetral and keep the engine going everytime I stop.

Tomorrow I plan on replacing the spark plug wires, maybe the O2 sensor, putting on that fuel filter, and maybe a fuel injector.

Basically, I was wondering if you guys had any thoughts.