firstly I would get the system powerflushed .The pump can be checked to see if it needs replacing irespective of whether it is turning properly by checking the resistance between the live and neutral on the pump(a good pump should be 220 and a pump on its way out could be 100 and lower) The system might be full of air which could be down to the way it is piped up.It is not the boiler because it goes well into ignition sequence but it may be blocked over the years within the heat exchanger
