Snipe,
Can you verify the pressure reading at the pump when the air flow stops? Do this without the diffuser hooked up.
What you are calling a flapper, may be the inside lining of the hose coming loose and causing a blockage.
The hose line may be defective. A warranty may be in order.
The positive pressure on the pump end and no restriction on the other, is a sure way to test the hose internally.
