Every once in a while the idle on my car will just go up to about 1200rpm. It'll stay like that for maybe 1-3 hours of driving and then it'll all of a sudden drop back down to normal. This has happened maybe 4 times this year. Unplugging the battery and resetting the computer only helped once. There are no other symptoms other than the high rpm. It drives normally and doesnt fluctuate or anything like that.. What could the cause of this be?