i have potterton 50e netaheat bolier stat keeps clicking?

My POTTERTON 50E boiler will fire up ok but after awhile that it has been working for I will hear the stat in the bolier keeps clicking every few minutes and nothing happens,this will keep clicking until at one point the boiler will stop.I would like to know why the stat keeps clicking every minute or so but nothing happen,is there a PCB fault.please advise