potterton suprima CH wont turn off intermittently?

About once a week my potterton suprima 60 CH goes mad and gets hotter and hotter. If i click the CH room thermostat down it keeps going, if i turn the CH off on the timer it keeps going. The only way to stop it is to power the boiler and timer off and back on, then its fine until the next time.

any idea whether its the boiler, timer, or thermostat?