I was driving the other day at about 50mph and suddenly the temp gauge went up to max and the power went totally. I pulled over and turned the engine off. I then started it again and continued driving. The car was fine until I reached 50mph and above then it happened again. I've had it on the diagnostics but no faults showed up. A mechanic replaced a sensor in the thermostat but didnt solve it then I had a replacement thermostat unit fitted and still no joy. The mechanic doesnt think its the head gasket cos theres no water leaks.Even while driving at 30mph, the temp goes up to half quickly without evidence that the car is overheating.Can anybody help, driving below 50mph is a nightmare and losing power is quite scary.