Straight gas AU1 - overheating / converter freezing.

[Cam]

So a friend has a straight gas AUI that has had some issues overheating (well, at least the gauge was rising), and the engine cutting out (temp gauge fluctuated pretty quickly). When cutting out the converter is frozen..

He's replaced the water pump, radiator, thermostat (now removed), rad cap, confirmed the thermos are kicking in as expected etc etc. the car has continued to overheat until tonight when it ran fine after the thermostat was removed. Was taken for a good drive and temp gauge didn't move at all. Just Around the corner the car cut out, and again the converter frozen. Car now won't restart. He said he's run water through the lines to the converter and it was all ok. The cooling system definately has pressure..

The rapidly rising and falling temp makes me think it's electrical (sensor in the back of the head, or PCM or ECU)

Any advice? Or next steps to take?