There are some may things that can cause this I hardly know where to start, potential causes:
Pump (possibly sticking)
By-pass valve (if one is fitted) closed.
Main thermostat out of range causing overheat.
Overheat thermostat faulty.
Fan pressure switch faulty.
Gas valve solenoid faulty.
BUT. The MOST likely cause is the pcb itself (dry joints) cause intermittent faults as described.
As many times as I have had this fault, it invariably comes down to the pcb.
There is a new version pcb available (unfortunately it comes as a pcb/wiring loom combined), it unfortunately for you, doesn't come cheap.
Carefree
July 2010
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