controller shutting down

[twill]

I have a honeywell central heating controller system (ST712 programmer). The controller shuts down if I attempt to have both the hot water and central heating running at the same time. The system works fine with either hot water or central heating on it own. I can switch between hot water and central heating using the controller switches. If I put both hot water and central heating on at the same time it runs for a while feeding both hot water and central heating after a short period the controller shuts down. I think the shutdown happens when the system is trying to change from feeding both hot water and central heating to just central heating. I know the acutator/valve is ok. Looking at the wiring diagram I think it could either be the controller or the cylinder termostat which is the problem, but am not sure which

[peter the plumber]

What kind of boiler is it?

If it were a combe boiler it would do that.

[twill]

Boiler is a potterton kingfisher mf. The system has worked for several years and this only started happening when I switched back from just using hotwater in the summer to combined hot water and central heating this Autumn,