Have you tried another battery? I had this problem with 2 of my batteries while the others give more flight time. I ended up modifying the phantom to take a slightly bigger battery and bought 4000 mah multistar batteries from hobbyking.
Thanks, Berio - Yesterday I bought a new battery (same model as the originals) and I'll see if the flight time is better - The store pointed out that my batteries were "swollen" and so might be older and worn out. We'll see. Thanks for your suggestion.