Alpha 240x shuts down,restarts, shuts down etc etc?

My Alpha 240x combi boiler wont stay lit for more then 20 secs then shuts down, it then relights some 15 - 20 secs later before shutting down again even under demand from the frost stat.Ive tried hot water demand only, heating demand only, both at same time with varying heat temperatures but no joy, any advice would be greatly appreciated.