truma e 1800?

i am haveing trouble with my blown air heater system.when i switch it on it can run without any fault for several hours. within the same day when i switch it on you can hear a tick as the burner ignites and then a click after about 10 seconds as it turns itself off.this happens often until you get fed up with trying it.it happens with both propane and butane gas can you help please

wevin
December 2010