Dear johnsargent,

Hope you're well and stay so. An invisibly thin layer of corrosion can disrupt an electrical circuit. Most common metals expand when warmed. So, a wire with defective insulation - that is warmed by rising temperatures and/or the passage of current - may expand just enough to contact adjacent metal. That suffices to disrupt the circuit.

When temperatures drop - or the engine is off - the wire shrinks enough so it doesn't touch the nearby surface: current flows smoothly. As the distances involved are minimal, these problems are hard to see/diagrnose.

Hope this helps.

Yours faithfully,

Spook