[oldE]

You have mentioned the cable routing by the radiator. Is it possible there are some hot spots due to flow restrictions in one area or another? Aging plastic or changing grades of plastic over time might increase the materials susceptibility to break down with higher temperatures.
Just guessing. I think at the least I would be adding insulating wrap until I knew why this problem occurred.
One other WAG :do you have an exhaust leak? I have seen a truck with an exhaust leak melt the air brake lines about a foot away and come to a screeching halt on the highway.
Best
Les