Is there any chance the RPMs turned out by the pump are causing the impeller to spin faster than it is designed to work at? (wild guess)
Impossible.

What about the impeller wear, I've checked it out and made sure it is clean, is there some symptom I might be missing?
I don't think that would cause the issue.

Sounds like an air leak but why it would be periodic if isn't a weir or water level, I do not know. Sometimes, suction side leaks will pool around the broken part of the pipe and so will appear to work for a while as the water is drawn back into the pipe but after than happens, it should just suck air.