when I was flying my nextar I had a few problems with the tank, having the wrong lines hooked up was one,also making sure the rubber bung was tight enough to seal the tank so there is no air leaks.
if you suspect air getting into the tank, a good way to check for leaks is submerge the tank in water and while plugging the extra lines, blow into one of em(some clean longer tubing works well so you can see if there is any bubbles).
another thing could also be the carb seal a small tear in the rubber can allow air to get in, hard to tell my FXi runs well years later but I haven't taken it apart since I have owned it.