F16N,
We have had the same results as you with electric water pumps. We just could not get enough water flow.
On the pulleys we build, we set the crank to pump pulley in a ratio of 1:1. This has worked very well on lots of FE engines. The problem we were facing was that most of the water pumps were being under driven and the engines would heat up while moving slow. At anything over 40 mph there were no problems. You have the opposite problem in that you are over driving your water pump by 35% (6.5/5.0)! I suspect that you are spinning your water pump too fast and cavitating the water pump impeller. When this happens the pump quits pumping.
Things that might help a cavitation problem... Run a bigger water pump pulley (or smaller crank pulley, but size it so your alternator still works), run a higher pressure radiator cap. Minimize any restrictions between the lower outlet on the radiator and the inlet to the water pump. An interesting experiment might be to remove the thermostat to see if you can stop the cavitation by removing this restriction...Something else you might want to try is to hook up a pressure gauge (an
oil pressure gauge will work) to your block and see what the water pressure in you block is doing...(hint with a centrifugal pump pressure should go up with the square of speed...)
Put your electric pump in series (between the radiator and the water pump).
Hopefully this give you a little food for thought...