whew!!.... either way, that truck pulling a 9k trailer in Vegas (no real grades) should not see that temp, not even on the hottest of summer days... no where close. Which brings to mind OP.... what does the harness look like going into the trans? Many of them leak and eventually turn the wiring insulation into jello which can result in contact between circuits. Just one of a many of possibilities. I tend to think this may be more of an electrical issue. Odds are that if you had something "going away" it would have reared it's ugly head on the Cajon or Baker grade. Please let us know what you find Tom

J