Rough idle

After the car is completely warmed up at an idle it has a slight roughness to it. Sometimes the idle falls and car nears a stall. Also upon revving the motor the engine doesn't immediately return to idle speed. It slowing will fall back to about 1500 rpm and hold there for a moment and then fall to the idle. I'm not sure what the problem could be. My car is a 94 T-bird SC.

Check the Throttle Position Sensor and have the timing checked.

Could also be a bad spark plug or spark plug wire if it backfires.