vectra 2.2 dti?

Hi, i have a 2.2 dti vectra. When the temp gets high, just before the fan kicks in, the spanner is displayed, the car is put in safe mode & eventually stalls.Left to cool the car goes fine as long as I keep the temp low. I replaced the ECT sesor & thought id cracked it untill it did the same the next day. Any ideas please!!!