I still have the missing under load and RPM on my 02 8.1. I have changed the fuel filter, installed new NGK OEM plugs and wires, cleaned the MAS (mass air flow) sensor, new air cleaner, and ran 2 cans of injector cleaner( SEA FOAM NAPA product) on a hard run. I still have the miss. I think it came on gradually. I have just turned over 100K and has never had any problems. I tow a heavy 5 th wheel. Any ideas????????