Hi,
Lot of good comments. I'm in the camp of air at the impeller (it's a centrifugal pump, correct?). The time-of -day correlation (heating the water) supports this. If true, then the fix is a 'blow-off' valve that you open while pump is running and 'air-locked'. Need to locate the valve as near as possible to impeller and at the highest point (where the air is). Manually (for now) open the valve, air will sputter out, then close as it becomes a water stream.
In a more drastic case you could partially close (force) a downstream check valve, simultaneously.
Best