206 Flashing STOP light...grrrrr

I have not had my 206 W reg diesel very long and it had been fine to
drive doing long/short trips. Now every so often when I am driving
along at 50/60mph I suddenly hear a clicking/ticking (relays??) noise
behind the dash and the STOP light starts to flash for about 10 seconds
inconsistantly. Temperature is flying up to red. Within 20 secs
everything calms down back to normal. This can happen any time between
me doing 40 miles and 700 miles!! Have taken it to main dealer,they
kept it for 3 days and
didn't know what was wrong and guessed they should replace the green
temp sensor & blue temp sensor but couldn't guarantee that was gonna
solve it. This obviously has not worked and it's still happening every
so often. I cannot afford to go to the main dealer again for a while.
Would really appreciate a reply from anyone else who has had a pug like
this or knows what it could be.


Bex