Sometimes after driving a few hundred miles at highway conditions when I restart the engine after a brief stop (like a refuel) the engine will race when I put it in gear and drive off. It will do this off and on for a little while and then act like it is running out of fuel. Sometimes it will die and sometimes it just lugs for a little while and then be ok. When it does die, I have always gotten it restarted and it runs fine after that. I have changed the fuel filter, but nothing else at this point.

What could be causing this?

 

maybe LP getting weak , sucking air some where

 

LP is going. If your looking to replace it, go with an electric one. Your still going to have to remove the stock mechanical, but the electric one, you can prime the fuel system way easier when you change the fuel filter(s). One more thing your going to need, is a fuel pump block off plate.