Does it have a mode button with 3 settings? If it has 1 of these is reset, turn it to reset for a couple of seconds and then back to on, this might work. Some Vokera boilers have a reset overheat button behind a circular button that unscrews on the front panel, try that if it has.The other thing is it may be cutting out because of low water pressure in your system, check the water pressure on the gauge on the front is between 1-1.5 bar. If no good after these there could be any number of faults causing it. Hope this helps.
Taliskerman
March 2011