Halstead Finest Combi Boiler Locking Out?

When a hot tap is turned on and the central heating is not on or on demand, the hot water flows whenever a demand for hot water is made - no problem. When the heating is on it will fire up but after a while the boiler cuts out and the red LED is on continuous, not flashing. When I turn off the electricity at the main switch, wait 5 seconds and switch on again, the boiler fires up. Seemingly this only happens when the heating is on and a demand is made for hot water - but not every time. It seems that instead of switching over to hot water the boiler cuts out. It sounds as though the fan is still running but cannot fire up. Despite having the boiler serviced, the engineer could not trace what is causing the fault. The fan was cleaned and oiled, air switch checked and overheat thermostat checked. Has anyone any ideas. This boiler has been trouble since it was installed in 2000. Halstead themselves replaced 2 thermistors, air switch and pcb in 2003 because of repeated faults which they would not accept as being inherent.