Temp gauge rising

Hi. I have a Peugeot 206. 120000 miles.

Why is my car stop light flashing on and temp gauge rising from 70 to red but car doesn't seem to be overheating? 2 months ago my car started overheating got it checked and had a new head gasket 3 weeks ago. But I'm still having problems when I'm driving The stop light flashes on ad the temp gauge goes from 70 up to the red. If I stopped the car and look under the bonnet there's no signs the car is overheating, water levels fine and it's not gushing out of top as was before had new head gasket.

The fan kicks in as soon as temp gauge goes up. Then if I turn my car off for approx 5 mins and back I again, the temp gauge has gone bk down to 70 and stop lights off. The no pattern with it sometimes it happens after 15 mins of driving and other times I can drive for over an hour with no problems. Any ideas? As I am reluctant to take it to garage again as have spent soo much on it in last month.