and how we can resolve it.

Tired pump?
Sludge?
Scaled up microbore?
Airlock?

You say they've stopped over time. How long are you talking about? A
week? A year?
Have you tried turning off all of the upstairs rads? What happens if
you do?
My next step would be to flush the system rather than the individual
radiators, maybe with some cleaning chemical. See what comes out.
If flushing doesn't work I'd reluctantly replace the pump I guess.
The age of the system shouldn't make too much difference in this
respect other than as an indicator that things might be wearing out.
That said the only thing I can think of which might cause this problem
because of wear would be the pump.

Good luck.

First it was the hall, just over a year ago, then the Dining room about a
year
ago, but now the two in the living room are just warm, the kitchen radiator
remains hot. We are going to try a power flush, if this doesn't work we'll
Get someone to look at the pump. Thanks for you help.