I had this problem a couple of weeks ago.
The lights on the dash say "STOP" and there is a light on the thermostat when the engine gets to 97ºC and the main fan cuts in, you pull over, it stops saying "STOP" and then comes back just after you pull away again.
solution:
the rheostat for the secondary fan is broken, they're about £7-8. You need to open the bonnet, with a star screwdriver take the front plastic panel off, and to the left of the center there are 2 connectors. Take out the leads (remember which way round they go) and undo the bolts so the plug comes out. it is a metal 'U' shape, with a spring and a resistor inside. This part needs to be replaced.
If this happens to you, put the aircon on until you reach where you're going.