You can check spark output for each cylinder at the coil pack. Get all 6 wires loose before you start the car, then pull off each wire one at a time with the engine at idle. You are looking for 2 things a good strong blue spark, and the idle to change with every wire you remove. You do not want to grab too close to the end of the wire or the spark will go through you and "bite" you as I say. This will lead you to which cylinder is not contributing.