Ravenheat Fanned Combi RSF 82ET - Flame Failure?

My Ravenheat Combi works fine most of the time but occasionally it locks out with the yellow flame faliure light coming on. It hasn't been so bad over the summer with just the hot water needed. I saw the problem maybe 4 times in as many months. Now I am starting to use the central heating aswell the problem seems to be getting worse. It always seems to work for around 20 minutes and then the flame failure light appears and the boiler stops working. I then have to reset the boiler by switching it off. If i then leave it for a few minutes and try it again, it will fire up again but after another 10 minutes or so it stops working again. I recently had the boiler serviced and i did mention this problem to the plumber but he said he everything looked in good condition inside when he serviced it. Any ideas anyone? Many thanks, Andrew