What is wrong with my Potterton Netaheat Boiler 60E?

I have a Potterton Netaheat 60E boiler. When the heating is programmed to turn off, although the light goes out on the control panel, the heating sometimes remains on and the house gets extremely hot even though all the radiators have thermostats which are set low and our main thermostat is set on 19-20. I have also found that when the heating does switch off properly it sometimes comes back on randomly and again is extremely hot regardless of thermostat settings. Occasionally the boiler turns itself off - sometimes it overheats and its a case of pressing the re-set button underneath. Other times I have to turn everything off and on again to restart. Does anyone have any ideas?