Many times plug wires will wear through, or get a bad spot in them. Generally this is only noticeable under load, not in park/neutral. Engine will run fine through all rpms, until put in gear, then will miss/bog/die. This is preferably a 2 person job. put on the parking brake (if possible) Open the hood. Get a spray bottle with water in it, you want one that can do a nice mist. Have a friend start the vehicle, hold the brake solid, and put in gear. Lightly mist over the plug wires. DO NOT TOUCH THE PLUG WIRES. WITH WATER ON THEM YOU COULD GET SHOCKED BADLY IF ONE IS BAD. If you have a bad wire, you will hear a snapping type sound, and possibly see spark arcing to a ground point (block, exhaust, head, etc). Change the faulty wire or set. I have also seen times where the spark will run down the outside of a plug with buildup of carbon, etc, and, while rare, can be tested using this same procedure (mist around the plugs).