boiler lock out on baxi bermuda 54/7?

i have a fault on my boiler, the boiler will light up ok and some times it will run for a few hrs then lock out.then re set boiler it may run for few hrs or a few nins then lock out again. i was told this could be a faulty circulating pump, not getting rid of the heat in the boiler then tripping the boiler . i thought that this was controlled by the thermostat. any info please