High reving by itself, weird shifting

My 2000 Pontiac Sunfire, 2.2L 4 door has over the last week or so started to do some weird things.

It usually happens about 20 - 30 mintues into my 45 minute commute (It has happened 3 times of the last 4 trips). At which point the car will downshift and begin to rev very high, regardless of what I do with the pedal.

When I shift into neutral the RPM's will redline with nothing touching the gas pedal.

You can feel the car start to accelerate on it's own, but while in gear it is controlable (usually won't accelerate past 60KM by itself), and if you hit the brakes it will slow down but it feels as if you are two footing it and still pressing on the gas, and as I said if you shift to neutral (or park) it will redline. Shutting the car off and restarting it causes the RPM's to stop redlining but it will still be reving high.

If at a stop and you restart to stop the redline reving, you can shift into drive and reverse and back without it starting to rev uncontrollably again, however the car is idling very high after this happens (normal is usually 600-800 rpm, versus 1500 after this). If you drive even for a few feet however the car begins the redline reving again.

However if you leave the car for a few hours it seems to drive normally again, and I have never had a problem on a trip less than 25 mintues long.

It's an old car, with a lot of K's on it and I don't want to spend a fortune fixing it.

The engine light has been on for the last 2 years after the Fan seized, but since everything is highway driving on this car it has not been replaced.

Any ideas would be very helpful, thanks.