I would open it up and check that the valves are seating properly. Given that it is working intermitently it sounds as though this is the most likely problem. I would also check the boiler clack valve is seating ok as this can interfere with the performance of the pump if it is not working correctly. A second non-return valve somewhere in the line from the pump is always a good idea if one isn't fitted.
Hope this helps,
john
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