What's wrong with my Worcester 350 combi boiler?

My Worcester combi sometimes shuts down just after it lights to heat the water for the central heating. It seems as though it lights for up for 30 seconds or so to a low temperature and then shuts down. If I reset it manually a few times by pressing the heating advance button this sometimes gets over the problem. This fault only effects the central heating not the hot water.

Ken
October 2009