potterton boiler short cycling ?

I have tried many things to stop my potterton prima f 100 boiler from cycling (starting up for 10sec then turning off) when programmer is not calling for heat or water ,i have got a timer switch on the supply to boiler which cuts all power to boiler when not needed otherwise it will cycle every 20 mins constantly , tried overheat thermo, boiler thermo, room thermo, tank thermo , motorised valves ,maybe the system is not wired correctly ,any ideas ,