When I set custom white balance in the camera, I don't seem to get consistent colours across a few photos. It appears that the green/magenta slider (in Lightroom) is set to a different spot each time. Why is this?
There seems to be a lot of confusion about your white balance variability, and your experience does not agree with mine.
I just reviewed a series of 140 (selective images, perhaps 20) shots of one of my son's basketball games taken with the Nikon D3 with a pre-set read off of a WhiBal card. The illumination was metal halide. The ACR white balance reading was 3450 tint -3 without any variation at all. In another series of shots with flash and the WB on the D800e set to flash WB, all images gave WB readings of 5650K tint +16. Another series of shots with the D800e with the camera set to sunlight, all readings in ACR were 5050K tint +8.
If you set the camera to auto, there will be variations according to the scene and light conditions.
In another series of shots just for this post, I set WB on the 800e to 5500K and took shots under daylight and tungsten illumination. The ACR WB of all shots was 5340K tint +6, with no variation whatsoever.
Perhaps you could post a few rawfiles for others to check (use Yousendit or one of the Cloud sites that permit sharing of large files such as Microsoft Skydrive).