OK there are 3 reasons 1 the boiler fires dose the boiler have a pre heat hot water mode ? if yes then this right
2 the water gos hot then cold, you have a blocked heat exchanger or it scaled up clean of replace, 3 presser gos up it could be your filling loop is open or letting bye check or you need to put air in the expansion tank good luck
capitalboilers.co.uk, June 2009