On removing (for other reasons) fuse 56 in the trunk (boot) fuse box on my March 2000 built M5, when I put the fuse back the Nav screen flickered intensely, especially at engine idle. I have read about this problem on the board, but not experienced it before.
Again removing the fuse and putting it back in solved the problem.
My conclusion is that any tiny resistance in the power wiring for the Nav display can cause screen flicker, and that cleaning the fuse conections can help.
David