vokera 80 excell?

my boiler is running hot-colder-hot water. When I just start to run it boiler goes on(fire inside it) run for 30 sec or so and then switch itself off on pilot(water goes colder) then switch itself on(fire goes on full again) and repeat like that all the time, while central heating works fine it seams.
What is the problem with it???
Thanks,
Renato.