i have an 8 year old biasi which up till now has been fairly reliable but now getting various faults - boiler firing up on c/h then running until it overheats and cuts out sometimes not cutting out untill after very hot water is expelled - pump running but won,t fire up till i run (very slightly) hot water - boiler firing up but then firing on and off before temp rises much have checked diaphragms ,pump,changed thermisters any ideas ?