Netaheat 80E goes on/off?

I have a Potterton Netaheat 80E. As far as I understand the beast it is a gravity system and has been serviced by a fully qualified Corgi gas engineer. I am happy with everything that he has done and that is not where the problem lies.

Everything was working fine until the pump failed and this was replaced. In order to do this he had to drain the system as the shut-off valves either side of the pump could not be undone – so he replaced the pump and 2 new shut-off valves.

Water turned back on and system re-filled – radiators got hot and plenty of hot water from taps.

That’s when the problem became apparent.

The boiler starts up and after the initial burn [15 mins] it shuts down and then starts again after a minute or so – then goes off – then starts again and burns for a minute or so then shuts down and then starts and then shuts down etc etc. It then just recycles like this.

We have now changed the room thermostat and the tank thermostat. I am told everything else is working just fine but neither he, nor I, can understand why it has started to do this.

He is suggesting that the PCB be changed but I don’t want to continue to change everything piece by piece if someone has either experienced this and knows what the problem is or actually knows how to cure the problem.

I hope I have given enough information – obviously the boiler is 15 years old but everything was fine until we changed the pump and drained the system – all suggestions gladly received to help excessive head-scratching currently going on.

Thanks everyone.