I took my truck out to Montauk last week where it sat for a week once out there. Made it there no problem, but on the way back I started to here like backfire and CEL started to flash. It then just turned on. When I got home I scanned it and got a p0301 code. It doesnt seem to do it whike at lower speeds. Only at highway speeds when I get on it. Could it just be plugs?

 

Pull a couple, see what they look like. If they are old, just replace them.