Auto trouble

I have a 96 mercury mistake (mystique)and had trouble with it overheating and the rpm while idling had delined excessively & sometimes would stall out @ complete stops. I took it to a auto shop and they replaced the timing belt and waterpump. This was suppose to fix this problem however it continued to idle at a lower rpm then normal so then it was diagnosed as having a bad idle air control motor, I was reluctant to let them do any further work as it is starting to cost lots of $. What does it sound like might be the real problem here?