Hi Wibble, thanks for posting this - I think I know what causes this. I think I should be able to fix it but it will be just a little bit before I'll be able to get to it.

If this happens, sometimes placing the profiles in different locations can help - in this case using just the top one and not the bottom one gives a good result.

In the broken case, there is a bit of bunching going on that is creating a little wiggle in the surface, the shading is actually correct, the thing that is broken is that the surface actually has a small speedbump in it there.

- Michael