heating going off before its time cicle?

when i set my heating for one hour or more aftr about 45 minutes the heating goes off. also i cannot run a bath whilst the heating is on as the water goes cold after about 5 mins and the heating goes off. last year i changed the temperature htermistors and the heating was ok. but after 12 months it has gone back to how it was. do i need to replace the the thermisters again after just 12 months or is there something els wrong

terry
November 2011