When we had our gas rig we had the same problem.
I noticed it would fill pretty well until it got to about 1/2 full then it would start what you described, cutting off and get really really slow.
What I found was that the fill pipe did not enter the top of the tank instead it entered the side at approx 1/2 full level. I decided that when the gas in the tank covered the fill pipe the gas entering after that had to lift the gas in the tank resulting in a slow fill
Sorry, I never did come up with a fix I just lived with it.
