What causes the STOP light to come on and the temperature gauge to drop?

The temperature gauge shows that it is within the ideal heat, then it suddenly drops to zero and the STOP light flashes on the dash.
I turn up the heat and put on the blowers and it returns to normal and the light goes out!?!?!?
Any idea whats up, anyone! There is no particular time or reason this seems to happen. Some journeys it does not happen, others it does several times.