My personal car  recently developed a randum missfire when I turned on the A/C.  Since I had recently replaced the plugs, cap and wires I figuered it must have something to do with the fuel system.  I went to my local parts house and got a bottle of fuel injection cleaner and it has taken care of the missfire. If you have a vehicle with 100,000 miles or more, you might consider adding a bottle of fuel injection cleaner.