I have a 99 suburban that has me puzzled. It runs great but when I go up a long steep grade like the grape vine it misses bad. Ill be going up and it starts shaking and jerking back engine light goes on and it running good after that. Now I changed the cap rotor spark plugs and wires thinking it needed it and it still did the same thing. Then I got a scanner put on it and it said cylinder 4 misfire. So I changed the plug on that cylinder again thinking it was defective and the wires again but did the same thing. Now it only does it on steep long grades in the streets it smooth as can be and on a regualr hill its fine. I can pull a trailer on the freeway with a load and its fine. It only does it on long steep grades. 
