1997 Chevy Lumina misfires, stalls

My car has been acting strange for a couple months now. At first it had little to no peddle response in overdrive at high rpms. The problem has recently gotten considerably worse. Now my car is misfiring at idle and stalls out if I don't keep it at high rpms (3-4000) and feather the gas but when I just try to let it come back down to idle it dies. The diagnostics computer gave me the multi-cylinder misfire and EGR valve codes. I replaced the EGR valve, the spark plugs and wires and I've had the coil packs and ignition module tested and both check out okay. I've also had it to multiple mechanics who tell me they don't have a clue. Can you help me? Thank you.