It drives well, but it will not idle. What could be the problem?

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I have a 1997 Geo Tracker, 1.6L, automatic trans with 120K miles. The car drives very well. Except, for the last few days, once it gets warmed up, it will not idle. At this point the car will continue to drive well as long as it is moving down the road. However, it will not idle. It normally idles when warm is 800 rpm when at operating temperature. Now when I slow down to a very low speed, or stop, the engine will continue to low rpm until it reaches 500 rpm. Then is stall. It runs well until it stalls, no roughness to it. If I am able to catch the accelerator before it dies, it'll run rough for a brief moment, then run well again. Once it stalls, it will immediately restart if I keep the rpm to about 1K. But the problem remains until the engine cools off. A shade tree mechanic suggested it is probably the idle position sensor. But at $184 for parts, I'd like to make sure before ordering parts. Any ideas? I'm on a limited income and have very little money to invest. I want to get it right the first time