2001 Buick Skylark car starting problem

2001 Buick Skylark mileage: 55,000. I have been having trouble with my car for a while it starts up in the A.M, but when put in gear it will stall. Then it will allow you to drive a little ways then stall, it will do this until it is warm. Now it seems to be idling hard and hesitating/missing, feels almost as if it is running on 3 spark plugs instead of 4. The alternator, starter, muffler, oxygen sensor, coolant sensor, EGR valve, fuel filter have all been recently replaced to try and remedy this problem. The only thing that showed up on the diagnostic done recently was the EGR valve, this didn't help, the minimum airflow was also adjusted. Please help everyone is stumped - could this be the catalytic converter (I think it could be).