What's you gas mileage like? If bad - check O2 sensor. If good - Run some injector cleaner through - could be caused by a bad spray pattern with your injectors causing the fuel not to vaporize properly. If injector cleaner doesn't work - have your injectors checked.I am assuming that you have checked your distributor cap for cracks, carbon tracking and pitting and rotor and sparkplug wires and plugs first.