First thing's first, you need to adjust the pressure and check the expansion vessel, when cold you should have 1Bar of pressure in the appliance...
The expansion vessel should also be at 1Bar, but you need to empty the boiler of all of it's water to check this accurately.
If the appliance is low on water, it can sometimes pull air into the heat exchanger, causing the appliance to overheat and trip... By what you are are describing, the underlying cause will be that the expansion vessel has lost its charge.
GasAid.co.uk
December 2006