I would go to one more mechanic and have him again check the vacuum hose which goes to the air intake and runs near the water pump area....

I just replaced my water pump, had the exact same Idle problem...took it to Benzmac.....he found the hose I disconnected by accident while getting to one of the water pump mounting bolts.

By the way.....anyone who lives in the Atlanta area should definitely go for service to Benzmac (Donnie and Bill have a first class operation and are very good to work with)