Sorry it's not much detail but i thought an experienced engineer would recognise the symptoms.
There is a gauge on your boiler that when it reaches a certain temperature signals the electrical thingamabob to open the gas to the boiler and heat it up and close the valve when its heated to a certain temp. It sounds like the gauge is gone wacky, call a plumber or heating tech that services boilers, cause it sounds like the gauge has gone haywire.
Wish you would put make and model when asking these type of questions. Many boiler have there own little problems some more than others. If the system has a bypass on the heating side then the water can return to the boiler without the full system getting up to full working temp. Is the boiler a PUMA 80 /100 if so it could be the wax operated diverter valve this will make the rads sluggish to warm up and also affect the DHW. Next I would look at the water temp sensors, these make the boiler modulate the heat output some in a stepped fashion (High, Med. Low, Off ) they can go strait from high to off in one step . A low output on the pump will give a similar situation the water is heating up faster than it is passing through the boiler. One other thing to all that post questions of a technical nature would be the location UK USA etc. things are so different across the water. CORGI Reg. engineer
