It could be something as simple as a twisted gas hose. I'm talking about the hose that runs from the external fill point to the tank. Mine was twisted, and after a slight turning to straighten it out, the problem was solved.
ETA: OOPS! Didn't read all of OP's post. Should probably replace filler neck assembly.
Last edited by ValleyGunner; 09-10-2013 at 03:27 AM.