Hi, I have a HK 500gt that I'm flying with the following setup:

Motor: Hobbymate 3650 1700kv
ESC: Mystery 80A, timing set at medium
Battery: Turnigy 4s1p 5000mah 20c
Servos: Turnigy MG930

My problem is that I can only get about 4 minutes of hover/flight time before the tail starts to wag and I begin to lose power. I tried the new nano tech 3000mah 4s1p battery as well, but I got only 3.5 minutes before the tail wag started. When I measure voltage of the 5000mah pack when the tail starts wagging, I have about 50% capacity remaining. Is there something wrong with my setup, or do I need to change something? I would really appreciate any advice, as I should be gettting about 7-8 minutes of flight time with the 5k pack.