Boiler cutting out

Hi, my Potterton Gold FSB 30 HE boiler has recently started behaving oddly, which is something else for me to worry about.

It will seem to be running fine, modulated down to a whisper, then will suddenly ramp up to what I presume is full power and will then switch itself off almost immediately. The radiators at this stage are just warm. Then after a few minutes it will start again and be ok for maybe 5 minutes, then it will happen again. The boiler doesn't display a fault, no reset is needed, but operating like this it takes ages for the radiators to heat up fully.

The pump seems to be running ok at all times. The radiators do heat up eventually, all of them, and there is no air to bleed.

I wondered whether it could be the thermistor, but the pattern of behaviour is hard to reconcile.

Can anyone suggest what might be the problem?

Many thanks

Never assume because the pump is running, it's moving the water. What temp is the pipe either side of the pump?

Hi, thanks for responding. The pipes are hot both sides of pump, but they would have to be, as the whole system is now up to temperature. But the problem still occurs - every five / ten minutes the system revs up and then cuts out. If it might be a failing or blocked pump, I will check tomorrow whether it is rotating freely (its a bit too hard to get to to do it now). Could that account for the symptoms I am seeing? Its the revving up but that puzzles me.

I'm not very 'au fait' with the finer workings of modern boilers, but I was thinking possible circulation issue, if the pump is failing to move the water away from the boiler, it would account for the rads taking ages to warm up, and the boiler cutting in and out as it gets to temp very quickly. Could be faulty pump, could be a blockage in the system, or I could be entirely wrong!