87 or 89 gas will most likely not cause the problem you are seeing, this is a classic symptom of a sticking idle control valve.

Another possibility is the air flow sensor potentiometer. With car off, check resistance. When the air flow sensor's plate is all the way to the top, you should get between 1 to 2K resistance. Now gradually push the plate down, the resistance should increase smoothly without any jumps, until the plate is well depressed. If you see jumps (specially high resistances), you need a new potentiometer, which now-a-days is hard to find, atleast a new one is hard to find. A high resistance jump causes the engine to rev up because the computer thinks you are getting a lot more air than you really are, and so sends in a lot more fuel, thus enriching the mixture.

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Saumil S. Patel