Turn off hot water and all downstairs rads as well except one that doesn't work. Hate to say this, but shut the bypass (if you have one) in the airing cupboard just after the pump.
Pump should not be getting 'red hot' to touch though.
If above doesn't work I would personally replace the pump.
Sort of in the process now of turning the downstairs rads on one at a time and the all seem to work .... just not when all on together.