When central heating starts up temerature goes quick up to 85 degrees on gauge and burner turns off radiators are only warm not hot. It does this several times and then evenutally goes slowly up to 85 degrees and heats the radiators to hot as it should do. The water pressure is ok and there is no air in the system. Also when hot water is running the burner cuts in and out and the water goes hot and cold.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.