Last night after normal driving, I put the car in park and left in running while I jumped out to get the mail. When it was idleing, the engine revs would go from about 750 up to around 2000 and immediately fall again. It did this for about 2 or 3 min. I got in the car and brought the revs up to about 4K and let them fall back down. This fixed the problem and the car was idleing fine. This morning, the revs were bouncing around between about 750 and 1100 when idleing. Any thoughts? I'm not due for an oil change or anything for another 1k miles...

Thanks in advance