sure it s not the rubber liner in the fuel line ? or sender in the tank breaking down . i built a 489 engine some time ago for pro street were the braided line linner was coming apart . for your fuel run the good stuff . put some stabilizers in it when it sit .i learn that the hard way myself on my car. would hardly run had to drain it gass was not old but the newer fuel must not be to hot any more for shelf life .the new fuel s must have some nasty stuff in it ? if i go thru may fuel lines nexit time i going all teflon braided the rubber gasket in the old holley blue pump was breaking down as well come to think of it