Try the obvious, and just open up the needle valve a few clicks and see if the problem goes away.

If that doesn't work, it could be an air leak inside the tank, in the clunk line. When the tank is full, the engire clunk line is in the fuel, but as the fuel level drops, the leak is now exposed to air and sucks in bubbles. Depending on the tank, this can eaisly happen where the clunk line attaches to the tubing that goes though the stopper.

Also check to make sure the clunk is not stuck against the front of the tank.

Make sure you don't have the fuel lines reversed. Usually you get shorter than 5min flights that way, but it's worth checking.

Easiest way to check is to see if you can pump the tank empty. If you can't empty the tank with your fuel pump, then you know you have a problem.

It could also be fuel foaming.

Next time it happens, take the plane back to the pits, and re-start the engine with out adding more fuel to the tank. Take a close look at the fuel line. Do you see air bubbles? Pick up the plane, and hold it nose up. Does the engine keep running with either a slight RPM increase or no change? If you notice the RPM drop off, or the engine quits, you are too lean on the needle or have some other fuel draw problem such as the ones above. Usually it's too lean on the needle valve.