Hi Micheal,
I do appreciate that it's hard to toubleshoot without having your hands on the offending data - I'm just awaiting permission now to send a sample STEP to you. NDA's etc.

And thank you for the advice. :)

I'd not heard of CAD exchanger - that was helpful, thank you! Interestingly, it looked as it should in CAD Exchanger, though no matter what STEP version I put out from it, the same errant meshes happened in MOI.
Also: MOI simply hung when trying to load a version that was output as .3DM (regardless of what version I set it to).

Again, I know this is of little use to you until you recieve a file - will get one to you asap (monday at earliest, by the looks of it) so no rush.
/RH