It could be corrosion on the impeller on the primary water pump (the one on the engine block, which is driven by the same belt as the alternator).  I assume that basic ideas like thermostat operation and specifications have already been checked? Otherwise you could suspect scaling up of the water galleries, especially if repeated overheating has encouraged topping up with tap water instead of antifreeze solution.
On my BMC1.5, I have fitted a pressurised overflow tank, so the heat exchanger stays 100% full at all times. The spring loaded pressure limiting cap is on the remote tank. I found one that was originally made for a Rover 2600, but the same thing was installed on many other cars.