The problem might be with the Idle Air Control valve. This is what controls the engine idle speed and can malfunction where it goes to the wrong position for the idle condition. The IAC valve is located on the throttle body and looks like this http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?displayName=Idle+Air+Control+Valve¤tPage=1&categoryDisplayName=Engine+Management&navValue=15900207&itemId=207-0&parentId=59-0
Start the engine and while it’s idling take the handle of a screwdriver and tap on the IAC valve. If the idle speed changes when doing this, replace the IAC valve.
Tester