I've found that there are a couple of things that can cause intermittant cooling water flow. Here are some things to check for: Make sure the cap to the seawater strainer is well sealed. The pump can suck air from a poor strainer seal. On the Perko strainer, it can be tricky to screw it down level, and the gasket is cork and after years doesn't seal as well. Also, check for wear in the water pump housing and the little cam that compresses the impeller blades. It is easily replaced and wear reduces the pump's effectiveness.
