2003 Buick Regal rough idling

I just got done with a head gasket job on this vehicle.and thanks to you guys with your help about how to resolve a rocker arm and pushrod problem that I had.the car drives fine as long as you"ve got your foot on the gas pedal.however when you come to a stop,it shakes and idles very rough.has a check engine light that's on.and code is po300.radom misfire.so far I have replaced spark plugs cleaned injectors checked for vacuum leak the only thing that I have not replaced is spark plug wires.the problem has not disappeared.what else could I be missing.and also how come it drives so well on the freeway.,the problem only shows up when I come to a stop.tell me how I can send a donation you guys are well worth it.