For a while now, my 5.0 Mustang has been doing this tempremental high idle thing. Most of the time, it runs normally, but every now and then it decides to idle at high RPM. How high varies, usually 1000 or 1300 or so, but it occasionally does 1500, and one time it decided to idle at 2000(!). It doesn't do anything different when cruising along, but the RPM does jump a little when I shift, like if I shift at 2100, the RPM will go up to 2300 or so before I release the clutch. Normally, the RPMs stay the same or do down slowly when shifting, for a smoother shift, I suppose. It runs normally again if I turn the car off and then restart it. I also notice that when it's in "high idle mode" the RPMs gradually increase when I slow to a stop in neutral. It may drop to 1000 or so rolling in neutral, but it goes up again as the car stops. The really strange part is that this happens seemingly at random. It may do it in the morning or at night, engine warm or cold. It might do it as soon as the car starts, or halfway through a trip. I'm guessing this is some kind of electrical problem, but what could it be? The car has about 170k miles and still runs great (except for this idle thing).