idle sticks after certain speed

When going past 80km/hr the engine sticks at high idle. When slowing down, the idle remains high. It's almost like the car drives by itself without giving gas. When put into park, it is very loud until turned off. I have had a mechanic look at it and he said he repaced a chip or a sensor (can't remember) but the problem is back. Can't afford this problem at record gas prices. Please help.

I vote TPS (throttle position sensor). when replaced, it has to be adjusted to within a certain voltage. I can't remember which voltages, but you could search the Lesabre and Olds 88 fourms (assuming yours is a 3800, those forums have the same engine) since they get more traffic.