I'm looking for a little help with tuning my Quadcopter.  It's a little too big to tune while holding (or suspended in a jig) so I've had to resort to tuning it in the air.  I'm new to this whole process and have been having a really hard time getting my quad dialed in.  It flies in "stabilize" mode but drifts around quite a bit (I am constantly having to adjust the pitch and roll to keep it from drifting away).  Besides the drifting around I am also getting some sporadic twitching of the copter during flight.  I have worked very hard to balance all the props well.  Even though I've mounted my APM 2.5 on foam for vibration dampening I have chosen to load 2.8.1 Arducopter instead of 2.9.1 because I haven't really proven whether vibrations are low enough yet.  

Here's the specs of my quad...

APM 2.5 running 2.8.1 ArduCopter

3DR uBlox LEA-6 GPS

MB1260 XL-MaxSonar-EZL0 High Performance Ultrasonic Range Finder (with noise filter resistor/cap)

Dual AEO U5 5V/5A UBECs (one powering the APM 2.5 and one powering LED Nav lighting) 

RCTimer SK-30A SimonK Firmware Multicopter 30 Amp ESCs.

RCTimer BC3530-14 1100KV Outrunner Brushless Motors.

RCTimer 915Mhz Telemetry Radios

A two axis carbon fiber camera gimbal with a Sony Action Cam mounted.

29 1/2" Prop shaft to Prop shaft diagonal width.

11 x 4.7 Props.

Powered by one 3S 6000mAh Turnigy Nantech LiPo  (I get roughly 15 - 17 minutes of casual flight and 11 - 14 minutes of more aggressive flight). 

RC Gear - Spektrum DX6i with 6 Channel Rcvr

Home built frame made of 3/4" square aluminum tubes and two 1/16" thick 6" square aluminum plates.

The Quad is set up in an "X" flight mode.  The default PID values were where I started and I've been adjusting away from them with little or no change to the flight characteristics I described above. Here's a screenshot of where I am currently with the Arducopter tuning values.  

Here's a photo of my Quadcopter for reference.

I don't really know how my copter compares to the 3DR ArduCopter Quad setup as I haven't really compared the specs.

Any help that the members of this community can provide would be greatly appreciated.

Thanks,

Ken