What causes intermittent loss of prime in a pool pump?

A suction-side air leak is almost certainly the problem. Common suspects in order: pump lid O-ring (dry, cracked, or not seated), skimmer basket weir stuck open, water level dropping below the skimmer, or a tiny crack in the suction pipe. Check the O-ring first—lube it with silicone, make sure it’s clean and snug.