Boiler starts up no problem for CH, radiators get warm, after a few mins boiler starts knocking and banging, undid screw on front of pump to check running but found pump had stopped (impeller spins freely), switch boiler off to let cool down for maybe 20mins, and boiler starts again no problem but same problem starts over again when pump gets hot. any ideas please. (boiler flushed a year ago and new divertor valve fitted.)