Trouble Shooting Question w video: Will leak cause cavitation?

[AlgaeFarmer]

mas985, Thank you for your reply. The weir operates correctly and the water level is good. The skimmer pot is not emptying. In fact, both the skimmer pot and catch basket on the front of the pump-housing can be filling when the pump begins to cavitate.
Upon first starting the pump it will prime itself, the catch basket will fill completely, the sound of the pump becomes a nice hum with only a single small bubble or no bubble at all under the plastic cleanout top. About 30 seconds later the impeller begins to whine then whitewater under the cleanout cap, then the circulation slows, but water is still entering the pot throughout this process. Circulation continues within the cleanout pot, but is slowed and slowly rises while spinning until primed again...and repeat.
I've replaced exposed PVC on the wet side and on the push side from the pump to the sand filter, lubed the cleanout cap and o-ring, even replaced the front half of the volute where there had been a stress crack. Tightened my tension band around the volutes to 1/4" per the user's manual. A pool guy came over to troubleshoot yesterday, advised the PVC work that I did and mentioned the possibility of a leak underground.

At this point I've looked at this thing so many times that I'm starting to see anything that is POTENTIALLY wrong.

I'm not writing off the possibility of an underground leak, but the problem with that is that water flow never stops (actually there are rare instances where the line will "chug" once or twice and immediately begin to flow again, but this is rare)

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)
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've also checked the large o-ring in the volute seal and the smaller O-rings at the intake of the volute and tightened the tensioning band.

Even the professionals seem stumped.