Hi Marc, this seems to be a popular shape recently!

It can be pretty tough for the boolean code to handle shapes that criss-cross and barely graze each other, and the more added the more complex things get. It's especially a problem with curved type surfaces rather than straight ones, because the curved ones tend to end up with a lot of little tiny slivers where they cut though each other just barely.

Please check out Danny's message here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1792.12

for an alternate construction method that will be much more reliable for building this shape.

- Michael