A well known issue is that the switch at the back of the ignition key cylinder is wearing out - this can cause exactly the behaviour you are seeing here. My SD has this issue from time to time. Why heat would trigger it beats me.

Symptoms and repair are well written up on the Diesel Giant site, ************** and indeed this forum as well! If the car is starting OK when you jump the junction box inside the engine compartment I do not believe it is a starter motor problem, but I would have to look up the wiring diagram to be 100% sure.

My belief is when shorting the terminals up front the starter solinoid sees more voltage than through the key switch and its associated circuitry. Enough more to activate it. The solinoid may be a little more stubborn when hot as heat increases the internal resistance. A good cleaning up of all the connections in the circuit may help a little. These are very old cars remember.
Changing the starter out could help with an old solinoid. A good interm soltion might be to extend the fender terminal wiring with suitable gauge wires back into the passenger compartment. On those occasions when the key switch will not activate the solinoid just have a push button in there. The higher voltage reaching the unit will activate it. First clean up all accesable wiring terminations in the circuit. That may be enough. Or measure for voltage drop throughout the system. You may discover a large or larger than normal loss somewhere.