Pump doesnt work when on and system banging

[KW3725]

My central heating is banging near the bolier, really badly.
Ive found that the pump doesnt seem to work when the heating is on so the upstairs radiators get hot but not downstairs, and the boiler starts to bang. Im thinking the noise is caused by water starvation to the boiler as its not getting water pumped to it.
The motorised valve works (water thermostat makes it move)
Heres the really strange part, when the room stat is turned down, switching off the heating demand, the pump kicks in and runs on its cooling cycle, proving it works!!
Anybody got any ideas??
Thanks,

 

[jacobball]

try vent the pump.
does the shaft rotate freely??
what is the resistance of the pump?

 

[twgas]

Sounds as though there is a wiring fault, possibly something to do with the motorised valve. Have you had any work done recently?

 

[gas4you]

It would help immensley if you posted make/model of boiler and system type etc.

 

[KW3725]

Thanks for your replies.
I checked the motorised valve and it operates when the thermostats are adjusted (so they work too).
I know the pump works as it runs on the cool down cycle when the heating is turned off, but doesnt work when I want it too.
The header tank is full and operates ok.
Later last night the boiler then refused to even switch on!
I had a think and bought a new pcb board for the boiler. Hey presto, its now sorted so Im slowly warming up and happy. The boiler is a baxi, around 10 years old.