My 1995 s 320 long was idling fine but when car was under load it wood start to miss i pulled obd generic codes from car random miss fires all six cylinders . changed all plugs and car run fine for about 4 weeks. car now idles ruff jumping between 600 rpms and 800 rpms , under initial load misses then is fine at speed 45 80mph have not pulled any codes yet friend has the scanner. the car has new harness .can it be bad gas