I believe that the problem is intrinsic in the camera as you found out, what I have seen over time is that some CCDs are more sensitive than others even in the same model camera. In low light conditions the CCD is less able to cope. In Yap with the VX1000 some years ago, I found that the best way of getting WB settings were to : as mentioned before , do it often on the way down and experimenting with pointing at the sand, the sun, a slate.... and for different effects ie. making Mi'l channel green turn blue to try a white balance on the pink/red coral polups.
Not Telling
