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Hi folks.  My APM 2.6-controlled QAV400 is having major issues.  I need your help.

The Backstory

I have a QAV400G that has been flying for several months on an APM 2.6 with beautiful results.  Most of these flights have been on beta versions of ArduCopter 3.2 (outstanding work, devs!). 

When I installed the gimbal and started focusing more on video-work, flight duration and stability became more important to me.  Since the QAV400 can only run 8" props at stock, I decided to put on QAV500 arms that would give enough room for 10" props - hoping to increase flight time and stability.

After installing the new arms and props (with no changes to PIDs) my first test flights were wobbly, but successful.  It looked like I just needed to run another AutoTune and manually play with some PID values to achieve smooth flight.  

On the same evening of those first flights, I put some DuBro foam under my flight controller (hoping to damp vibrations better) and I switched my control tx/rx to EZUHF from FrSky.  These were changes that I didn't think would make a significant difference in flight characteristic, but I was wrong.  On my test flight after the minor changes the quad rocketed into the air with just a bit of throttle, flipped over at 10-15ft up, and came down hard snapping one of my gimbal motors.

Today I tried to resolve the issues by retracing my steps:

• Switched back to FrSky 2.4GHz control - same problem, quad takes off very fidgety and then flips.
• Changed DuBro foam around and even removed it entirely - same exact fidgety-ness and flip.
• Upgraded to ArduCopter 3.2rc10, reset all my parameters to default - still fidgets and flips

I'm tearing my hair out here.  Does anyone have suggestions?

Thanks,

Brennan