It's not a 'check valve' problem; it sounds like the motor's thermal overload is kicking out, and then resetting when the motor cools.
CAREFULLY, put your hand on the motor (very careful -- it may burn you otherwise). If the motor is very hot, then you've probably identified WHAT is happening, but not WHY it's happening.
Possibilities include low voltage, worn bearings, changed plumbing, and more.
One more common problem: trash, leaves, etc blocking the air vents on the motor can cause the motor to overheat and cycle on and off.
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