I have a strange power problem. My IMU shield only seems to work when powered via the USB plug. When the APM is powered only via the BEC, the servos move erratically, the ABC and TX/RX LED's on the oilpan do not illuminate- although the oilpan power light IS on, the Power/MUX/PPM lights on the APM board are on, and the XBee power LED is also on. When I plug in the USB, the IMU powers up, and the autopilot begins functioning normally. If I then remove USB power it will work for 4-5 seconds, then begin behaving as above.
I've checked the board, and nothing seems burned out. Is it possible one of the capacitors is dead?

Thanks for your help!