My Potterton Profile keeps going out - Please help?

Help with Potterton Prima Profile?

Hi I wonder if you can help. I have the above boiler which until recently has been fine but within the last few weeks it keeps going out. As soon as I reset the overheat thermostat it is fine again. But then goes out again maybe one hour later or 1 day later or even 1 week later. I had an engineer look at it and he replaced the overheat thermostat. But got up this morning and it had gone out again. Pressed the overheat thermostat button and all ok again so replacing that part has obviously not fixed the problem. It also occasionally make strange noises. The engineer said it could be sludge in the system which would cost about £350 to drain and clean through or it could be the pump. I don't mind paying to get it fixed but dont want to pay out uncessarily if it doesnt fix the problem. I wonder if you could give your opinion before I call him back. Many thanks