I happened upon this fix on the internets and thought i would share my experience with the community here:

Do you have a Chugger pump that tends to screech and stop pumping at high temperatures? I sure did.
But the fix is pretty easy and seems to have worked for me.

Disassemble the pump head by removing the four screws holding it in place on the motor connecting bracket, then remove the four screws on the back side of the pump head itself thus freeing the rear housing plate. pull the impeller out by sliding it off the shaft in the housing body.

Then, using a 17/64" or G drill bit, carefully enlarge the hole in the center of the bushing. clean any loose material and debur if necessary.

Now you can reassemble the whole thing following the reverse procedure of disassembly.

You're fixed Chugger should work like a charm now. It worked for me. The terrible sounds should be gone and it should continue to pump like a champ right up to near boiling.