The car will normally idle around 700 rpm or so indicated on the tach, but sometimes when I come to up a stop it will stay at 900-1000, and at that speed its pushing against the torque converter pretty hard so it takes holding the brake pretty firmly to get it to stay in one place. If I keep my foot on the brake and rev it up slightly it will drop to normal, and the accelerator pedal is definitely all the way up when my foot isn't on it. It does it every time I drive it but usually only a couple of times, and the rest of the time it behaves normally. It doesn't seem to have anything to do with engine temperature or any other particular situation. Any idea what might cause this?