Did you make sure your fuel filler cap is venting properly? If it is not... it will pull a vacuum inside the fuel tank... after just a few minutes which will prevent fuel from moving freely through the fuel line.
Because you said it will usually crank after sitting for 15 minutes: This could mean the fuel tank equalizes some due to the vacuum pulling some air through the fuel cap allowing it to start and run only briefly.
Try loosening the fuel filler cap to allow more than normal air flow and see what happens.
"Just Like Iron Sharpens Iron... So it is that One Man Sharpens Another Man." Proverbs 27:17