Potterton Profile 50e keeps "tripping"

I have a Potterton Profile which runs fine most of the time, installed 1997 running on natural gas, fully pumped.

Every now and again, there doesn't seem to be a pattern, the boiler will not reignite. Moving the room stat tells me it is working and the only way to get the boiler back on line is to switch off the power for a few minutes, switch it back on the room stat clicks and the boiler ignites.
I have replaced the overheat stat and the pump over run stat. When the boiler is running it will cycle on and off under control of the boiler stat. Should I replace it as well incase it is an intermittent fault? Or any other ideas?

JimmyD007 said:

Every now and again, there doesn't seem to be a pattern, the boiler will not reignite

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Does the pilot light?

The boiler goes completely dead, switching the power off to the boiler for a few minutes then back on "resets" the boiler and it operates as normal. The pilot light only comes on just before the boiler is about to fire up.