vokera linea works for 20mins then gas jets go off and on continually?

Hi hope some one can give me a clue, my vokera works ok for about 20 mins then burners keep going off and on, about 6 times in every minute. this happens when the heating is on if i switch it off and turn the hot water on boiler keeps doing the same thing. leave every thing off for about 30mins boiler will work ok for another 20mins.
hope someone can help

Allan