This occurs sometimes after driving for over 20 minutes.Engine starts to idle rough with a little surge. Shifting to N and and bumping up idle engine runs fine. In gear, with A/c on it's worse. After stumble on accel it smooths out and seems OK, but seems to lose power requiring more throttle especially on hills. This does not occur at all times,weather doesn't make a difference. I just had a $2000 bill for inspection (Pa.)and wasn't expecting to put more in right now. Is it an emission related problem ? The exhaust sound changes when this happens and the engine labors at idle. And the kicker is that if you stop to shop, dinner,visit friends, it may not reappear ! I heard of cleaning an EPR port ,but haven't been able to find a thread or link. Need some help....Thanks