Vacuum leak somewhere...you are going to have to execute one of all the various methods for determing where the leak is, starting at the front of the manifold and its plastic down to the head ports till you find it. Can be as little as the plastic tube is loose on the intake to the manifold to a vally pan gasket failure. Your mileage is about the time this kind of thing starts happening. One other thing, sometimes cleaning the MAF isn't enough, you have to replace it. Just depends, as the computer doesn't know whether the MAF is sending bad data giving the lean condition, or there actually is a lean condition due to a vacuum leak. Lean due to bad MAF or lean due to vacuum leak looks exactly the same to the computer, and it therefore throws the same code.

One thing you can know for sure, it isn't an 02 or other sensor problem, as the computer has set both banks as lean. This means the problem is on the top of the motor, before anything divides to one bank or the other.