Low voltage BEC may cause POSITION mode unstable?

I just setup another quadcopter using PIXRACER, after checking the discussion list, I prefer use 1.4.4 firmware, my previous copter crashed with 1.5.5 on PIXHAWK LITE.

last evening I had a test fly, the copter fly very stable in stabilize and altitude mode, but whenever I switch to POS mode, even the GPS 3D lock well, the copter intended to flip and became unstable.

Because I found a voltage warning on the QGC, I measured the voltage and found VCC is 4.7V, the BEC output 5.0 when empty load, the voltage dropped with full load: PIXRACER +receiver+PPM encoder + M8N+telemetry

so I changed another BEC with 5.25V empty load.

this morning I had another test, even I changed the PID P from 0.06-0.13, the copter fly very stable in STA, ALTI, and POSITION mode.