I would look at the li-po batteries first. The other thing you can do is make sure the bearings or bushings in the motor are still good. Even my 2nd gen TP 1320mah packs only lasted about 150 flights each before the discharge rate was not adequate.

BTW, the Hack A20-20L, TP 1320 pack and 10x4.7 APC SF prop is an awesome setup. I have it on a 14oz balsa 3D plane and it's incredible power. I also found that by going with the TP 3S 910mah pack I could drop an ounce in battery weight, which of course makes it fly better and only lose 30 seconds of flight time in comparison. My 1320's are not getting used anymore the 910's are so good and they can handle the power draw from the 20-20 with no problem. After a 10 minute flight of 3D they pack, motor and ESC are just barely above ambient temp.