Saunier Duval Themis 23ME works in 15 second bursts

Hi there, the boiler will only provide water or heating in 15 second bursts. If I intervene by either turning off the tap or interrupting the electricity supply briefly, before the burner shuts off, I can get another 15 seconds of burner operation. The internal pump is set to intermittent  operation so nether air stat nor thermostatic radiator valves are fitted.