Don't overlook the comment on tank position, and remember the tank becomes more buoyant as air is depleted. One other suggestion- take a 1 pound ankle weight and clip it around the tank valve. It's not so much weight that it will screw up your neutral buoyancy, but may be just enough to "level you out." I really think this is the answer, because I think the issue is your tank getting buoyant during the dive.
DivemasterDennis
DivemasterDennis