Help! z31 driveability issues

Ok, i have an 85 300zx NA with a driveability problem that i cant diagnose. The car has a bad misfire at lower rpms under heavy load. it seems to happen most when the car has been warmed up and driven for at least 20 min. It seems as though it is running very rich as i am getting horrible gas mileage and it runs well above 3000 rpms. It has no codes stored in the ecm. I have installed a new cap and rotor, maf, plugs and wires, and fuel filter. The car has good compression, fuel pressure, and doesnt burn any coolant or oil.

I am running out of ideas so any help would be appreciated.

Thanks

sounds like an O2 sensor...

Thanks for your advice i will see if that fixes it and let you know