From what I've read several times on here it may be due to overheat.
Either the overheat thermostat thinks the water is too hot and shuts the boiler off incorrectly...
Or
The thermostat is working properly and the water really is getting too hot because the circulating pump is not drawing it from the heat exchanger quickly enough.
You may be able to check these yourself but a qualified boiler person may be quicker.
Good luck...
Peccavi, February 2009
