1991 Plymouth Laser (non turbo)

The car can sit and idle steady all day long at 750 rpm in PARK.  When car is warm and put into gear, the idle drasticly drops and idles rough. ( car doesn’t always stay running. )    The car has new timing belt put on by the dealer, new plugs, wires, module, and air cleaner.  Can you give me any ideas to try and correct my problem?  The engine is a 2.0 liter, non turbo.