Potterton Profile 50e tripping?

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

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 replace 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?

JimmyD
January 2009