Keep in mind that there are a lot of different things in the head that can be just a little bit out of adjustment and cause your problem. Those things will cause the motor to have to work harder and will shorten your flight time. P3 is right, your problem may be the motor, but I think I would carefully go thru set up before I would change the motor. One big culprit is the flybar paddles. They MUST be equal distance from the center of the head, and they MUST both be set to the same angle of pitch.
P3 is also right about the battery. 5 mins ain't bad, but I would expect to get at least 7 or 8 minutes, but you should always limit your flight to about 6 minutes anyway to keep from over draining the batts. Overdraining them will ruin them.
Donnie D