Engine dies at 90C when out of gear!

[Martyn]

Car starts fine. Runs OK at the beginning with temperature guage showing coolant temperature rising as normal.

Then, when the temp guage reaches, at what looks like, dead on 90C (194 F) and I change gear (foot off gas pedal) the engine cuts out. When it happens I can sometimes jump start if travelling fast enough or use ignition key to restart. No problem restarting the engine but will die if coolant at 90C and not in gear and foot off accelerator. If slightly below 90C after problem has happened, then car continues to drive fine until temperature rises to the critical level and it happens again.

I imagine its an electrical problem somewhere, but I dont understand what is happening or what to change.

Look forward to your guidance.