Any ideas at all as to what cuases this, rather nasty, bumpmap prob?

This happens when I set Geometry Approx SubD to 3 or higher; the higehr the number the worse the prob. The problem is also not limited to just this area. Somebody mentioned it could be related to clusters, but I had a thorough look at the clusters, even going so far as to dump them and set up a completely new projection so there are no edited UVs, no clusters at all , not even for weights, and the same thing happens. The mesh has all quads in this area, no inverted normals, nothing (i.e. as clean as I could imagine any mesh being) so it leads me to believe it's an XSI bug of some sort (I know of other ppl having this prob and ppl saying it's a bug, which seems like the only thing it could be)
Any ideas?
Thanx