Check the clunk to be sure it's not wedged up towards the front of the tank. Sometimes a "hard landing" will wedge the clunk in the front corner of the tank. When the tank is full, it flies fine, but when the tank empties out a little, the clunk doesn't always find the fuel, and it will die out on you. If you typically got 10-12 minute on a tank, and now you're getting ~5 minutes before your engine dies, that's a common symptom of a wedge fuel tank clunk.