[Fr. Frank]

Dixon (ESD) is right on.
The original tank uses a 3/8" pickup fitting, so the line to the filter should be 3/8". Any motor capable of pulling over 12 gph should be using a 3/8" fuel feed all the way to the motor, not a 5/16".

The culprit will almost certainly be the fuel pickup or anti-siphon valve. But that's not the underlying issue.
The service issue is the question of what is causing the pickup to clog or the anti-siphon valve to malfunction. You can replace the AS valve with a direct flow 90* fitting, and a new pickup line, which are fairly simple tasks.

The problem is that if you've currently got trash in the tank blocking the pickup or restricting it, it'll eventually fail again even with the new hardware. My recommendation is to go ahead and pull, clean, and test the tank AND put in the new tank hardware.