I expect you are right on the money about the heat. Underhood temps seem quite high to me on my '87 560SEL. Think I may need a set on mine and on wife's SL. I have had similar problems with my 7.4L Suburban. They run real hot (195 t-stat from factory) and tend to use up wires. Gets even worse if you tow since temps go even higher plus need for max spark under high load.