This only happens after about 15-20 minutes at a constant speed (such as in cruise control) - at say 75mph. When I slow down to pull of the highway, the car idles very rough - almost stalls. If I take off again and punch the accelerator (around 5-6000rpm), everything is fine. No check engine lights, etc. It's almost like some kind of intake or carbon build-up issue, because a quick punch clears things up.
Anything anybody can suggest would be appreciated.