What sort of thermostat do you have ?
It may be like my fathers boiler that is controlled by the thermostat with 4 different timed temperature settings, so effectively turning boiler off with room temp only.
e.g temp setting drops to 7oC after 11pm untill 21oC at 7am.
Hope this helps.
There are dozens of different types of boiler and various types of heating system with any number of different controls. You need to post more information about the type you have before anyone can offer any useful advice. I mean, it could be anything.
I think I have the same problem with my Baxi Combi 80 boiler. When I had it serviced in October I mentioned to the plumber that I had a problem with the CH - it seemed to heat for only a few seconds and then stop. Then repeat. It didn't do this previously.
He diagnosed the problem as something to do with the thermister and duly replaced it. He also turned down the gas a little because he was saying it was using too much. However, this seemed to make no difference and I found that the overrun time started to increase from just a few seconds until eventually the only way to stop the overrun was to put the electro-mechanical timer to "ON", wait 10 seconds and then turn it to "OFF". At the same time, the boiler started to make what I can only describe as spurting/straining noises during the overrun, coupled with the odd bang.
Now the problem has worsened (after a long run to fill a bath) inasmuch as despite the timer being set to "OFF", and the HW / OFF / CH+HW knob being set to HW only, and with no taps running, the boiler will continue to run the CH and give me hot water. The only way to stop the boiler heating is to turn it off completely.
I asked the plumber to return in January and upon inspection he thought it was a PCB problem and duly agreed to look at buying one and fitting it. That was 6 weeks ago and despite calls and messages left with him, he never came back. He even took my instruction manual so that he order the right part!!
The boiler was installed in December 2002 in my second floor flat. Water pressure is apparently good and the plumber said I was getting a decent heat into my water.
(Apologies for the long post and as you can tell, I'm rather clueless when it comes to CH!)