02 V6 Camaro starting problems...

Ok when I first got the car they had to put a new waterpump on cause it went out car overheated. After they changed that it drove good and I drove it about 20 miles home. The next day went out to start it and it acts like it wants to die but doesn't. It sounds like that for like 10-15 seconds and then goes to about 2000 rpm's and drops back to about 600 and acts fine. It only does it when you first start it. Any ideas on whats causing it?