406 2.0 HDI Overheating or is it?

Just purchased a W REG 406 HDI 2.0 at Auction - ex fleet 100,000 miles was running fine for a couple of weeks perfectly but:
I get a red "STOP" light popping up on my dash and my digital display say's ENGINE TOO HOT.
Just before this happens my Temerature is reading normal at 90c but as soon as it happens the Guage packs in and drops to 0c. 5 - 10 mins later the Guage Kicks in agian its back up to 90c and no fault is displayed.
Also while this is happening the fan is going on and off normaly and not running constantly which suggests to me its an electrical fault.
Anybody got any ideas?

the most common cause of this concern on the hdi is the engine coolant temp' sensor easy fix just to the left of the head with a green conector on it hope this helps :-)