I was having that problem that the engine would bog down (slight back fire), i changed my plugs and wires, but still had the problem, i went to car wash and washed engine, then it ran like crap, felt like i was down a cylinder, i was reading about coil issues, i changed coil pack ($65 @ autozone) and that fixed my all my problems. runs great, no more bogging down at high speeds. good luck !