I'm working on a piece of sheet metal that has a number of holes and bends and so on, though I've had this problem before.
The symptom is that after some number of 3D operations I can no longer select an existing 3D part to for further adds, substracts or slices. I can select other 3D parts, like the ones I want to use for add or subtract, but not the larger part I'm building up.
When I've had this problem in the past my parts were much smaller and I could afford to just recreate the part from scratch, but I'm at the point where my parts are too complex to spend hours on end redoing things.
Here's the relevant info --
TurboCAD Delux 17.0 (not upgraded --- yet)
AMD 64-bit running Windows 7