I've noticed similar problems with my truck. My dilemma is that the fuel pick-up on the sending unit are broken on both tanks. When my truck isn't level, or to the point where it begins to suck in air- it surges. Try filling up your tanks and selecting between the two tanks to see if it fixes it. Another problem you suggested could be the lift pump. This is also another problem area. They're not expensive, and once the vacuum pump is removed, very easy to install. Even if it doesn't fix it; you'll still have piece of mind knowing that it's new.