1997 Ford F-150 Rough Idle

This truck has begun to run rough and will die at stop lights or when idling. It drives perfectly when you are on the gas and keep it above 1.5 RPM's. I have just tried running 44K Injector Cleaner through it, so far no change. The mechanic changed the temp sensor, checked for vacuum leaks, and was unable to get a read from the computer. He is convinced it is an emissions system problem and not an engine problem. The Check Engine light has been on for over a year but our mechanic has said that it is not related to the current issue. This truck had a complete tune-up ~10,000 miles ago. Could it be the Catalytic Converter?

Any help is much appreciated.