The first one failed at around 90K miles, the second around 130000K, the third around 165000 and again this week at 167000K. I've changed spark plugs and plug wires. Always the same cylinder and the frequency has increased. I've checked the ground and the wiring harness for damage. Not sure where to go from here?
Heat is the big killer. The wires themselves get so hot that their protective coating melts and they short... not completely, but enough for the overall resistance to go down. I mean the wire inside the coil...If you've ever seen a coil of wire (transformer, choke etc.) you will see a colorful wire (usually gold, sometimes other colors like red - but translucent - looks like the wire is copper) that is actually a plastic coating over the wire that has to be scraped off to solder - or melted off.... Could be caused by a) being someplace hotter than usual OR b) running too large of a spark gap. The larger gap causes more voltage to be built up in the coil and the coil finds new and creative ways to arc. Note: theoretically it can be too small of a gap as well - weak spark, but more current flow.
Also note: the LS style motors had different coils at different times. Make sure you get the correct one for your vehicle.