Y6 - PID tuning or build/control problem?

I recently completed a build of a Y6 multicopter and it seems to be very sensitive to PITCH, but seems fine for ROLL and YAW. First off, I’m wondering if this is a common problem with Y6/Tri configurations, since pitch is controlled by a single arm?

Here is a video of its maiden flight - all of the sudden jerks are a result of me pitching forward or backward: http://www.youtube.com/watch?v=N6JPXl4gzyI

I’ve already done some PID tuning to get it to this point, and even unlocked the pitch/roll values from one another to allow a lower RATE_PITCH_P, but it doesn’t seem to have much effect on flight control.

My current PID values are:

RATE_PITCH_P = .0500
RATE_PITCH_I = .0005
RATE_PITCH_D = .0025

RATE_ROLL_P = .0700
RATE_ROLL_I = .0010
RATE_ROLL_D = .0025

Can anyone suggest if PID modifications are needed to allow for smoother pitch control? Am I on the right track, or is this a symptom of some other problem with the multicopter’s construction itself?

Here are some specs of the build:
UAP2 Y6 650mm frame / 880kV motors / 11"x4.7 props
30A SimonK ESCs / 3S2P 5000mAh batteries
Ardupilot APM 2.6 w/ external GPS + compass

Many thanks to anyone who can help with this!

turns out I had to enable EXPO on my radio - it flies great now!