At 120 k miles you should be considering replacing the spark plugs... its time. When the COPS are off, look at the rubber end over the plug and see if one is swelled 100% oversize. that happens if they get antifreeze or oil exposure long term. That COP you might replace. You can get a full set of 8 on e-bay for $100. and just do all, or do a few and have some for later. IF you play with the truck, stomp on throttle, back off, stomp, etc, eventually you might get a code, then you know which cylinder. In the mean time, i would consider all new plugs.