Radiators not working properly

[flipflop104]

A couple of days ago our downstairs radiators stopped working. They were fine until then and we only noticed it because it felt a little chilly downstairs. The upstairs ones are still on it's just the downstairs ones that are cold. They are currently set to be on 24hours a day.
Also, have noticed that the upstairs ones whilst still on most of the time (and set to be on 24hours) do go off and come on again randomly, periodically.

Any ideas what this could be or what could be causing it?

PS. We've tried bleeding them and it's not worked.

TIA

[woodchopper]

Sounds like one of the motorised valves if fitted has ceased to function - some have a manual overide to get you by until you get it replaced.
As to where its located - no idea.

[pilotlight]

The most likely reason for the problem is that the pump has failed. The upstairs radiators are warming simply because of the gravity effect of hot water rising.
Hope this helps.