bolier firing up when controls are off?

Help, my bolier keeps on turning on and off.I can hear the pump start up and then stop every 20minutes or so.When I check the boiler i find it running and and sometimes fired up then just dies out.The controls are all set to off and have to turn the power off to stop it. Have bled the system twice aswell.I can hear the pump start up through a slight click noise through the radiators. someone said it maybe the motorised valves sticking but they seem free and working ok when i checked

michael
October 2009