I have a 97 chevy 1500 v6 with the vortec 4.3 and 4 wheel drive. It would always misfire between 65 and 70 mph. It just recently started misfiring when it was cold for a couple miles and then wouldn't do it for the rest of the day. I check the plugs, the wires and the cap and rotar, all good. Now at work I was in the parking lot with the truck running and it shut it self off. Now when I try to start it it shakes really hard, turnes over but won't fire. Fule pump? Coil? Clogged cat?