Cylinder 5 misfire

I have a 2003 buick Rendevezous it has a 3.4 motor with 81451 mileage. My computer tells me that it has 1 of 2 codes in it cylinder 5 misfires, I had cylinder 5 plug, wire and coilpack replaced ant it still keeps giving me the same code. It also idles irratically especially when I come to a red light it I let the rpm's go below 1 1/2 the motor will die, but whenever you get up to speed you cannot feel it idle funny. It is driving me cracy. I need help with this issue. Someone told me I need a new motor could someone please tell me or at least help me with this problem