I’ve been plucking away at some work on my girlfriends car-1999 Chevy cavalier 2.2l. I’ve changed a fuel filter spark plugs/wires, and the coil packs. The plugs and coils made the car run a lot better all around but it still shakes quite a bit without a foot on the throttle, as well as won’t start unless you tap the gas. Was thinking of trying fuel pump next but didn’t make sense as the car runs great at all speeds. Any help is appreciated.

 

Welcome from Florida. Try cleaning the throttle body and look for vacuum leaks. Cracked/brittle vacuum hoses are a possibility.

 

If you have to tap the gas pedal it's running lean (getting too much air/not enough fuel) So:
IAC (Idle Air Control Valve) Clean/replace
Vacuum leak
Clean MAF (Mass Airflow Sensor) See YT for info
Check air cleaner

If none of that helps do a compression check.