Intermittent loss of prime is a suction-side air leak.

Yes, that’s almost certainly the problem. If the pump primes fine but loses prime after a while—especially if it happens off and on—you’ve got a small air leak on the suction side. The pump pulls in just enough air to break the prime, but not enough to stop it entirely.

Common suspects in order: pump lid O-ring (dry, cracked, or not seated), skimmer basket weir stuck open, water level in the pool 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. Also check the drain plug and any valve O-rings on the suction side.

If that doesn’t fix it, test by running a garden hose over each joint and connection while the pump is running. Listen for a change in pitch or watch the air bubbles in the pump lid. Bubbles = leak.

Clean the pump basket and check the weir flap while you’re at it. Weirdly, a stuck weir can cause intermittent prime loss as the water surface bounces.

If you still have no luck, you’re probably chasing a hairline crack or a buried pipe leak. That’s a call-a-pro situation.