Boiler cuts out before room stat temperature?

Hi
I currently have a problem where my boiler cuts off before the temperature reaches what the room stat is set for and sometimes it does not start up at all, also the hot water is really hot so i have turned down the setting on the water tank to around 35 and it is still too hot, but then i find the boiler does not start up unless i turn up the temperature setting on the hot water tank again.the problems seem quite intermittent sometimes i think it has resolved itself then the next day we wake up and it's cold because the boiler has not come on.
Any help would be greatly appreciated.
Thankyou.