Pond pump runs dry after a few hours

I recently installed an external pump with a priming pot for my pond.
After a few hours of running I discover that the pump is still running
but there is no water flow and the priming pot is empty of water. I
can stop the pump, refill the priming pot, restart it and it will pump
water for a few more hours until it runs out again. I am confident
that the power to the pump has not gone out temporarily. I have also
verified that I can remove power from the pump and restart it just
fine, since I have a swing valve on the input of the priming pot that
prevents it from emptying. Any ideas as to what could be causing this?