By calculations I mean the calculations ArchiCAD does to display your model.
For most likely the same reason any city planning maps and such you download (at least in NL) are drafted in meters (1 drawing unit = 1 meter). Scale down your model from mm to m, to see if that helps.
I'm not aware of any benefits to converting to object for displaying things faster in 3D, but maybe someone else can confirm this. In floorplan it helps if you make a simplified 2D symbol.
The stacked slabs do have more surfaces than a mesh would, even though you do not see them. You could try converting them to a morph and merging it to one morph.
I would indeed upgrade GPU and PSU, though there might still be other hardware failing, but a failing GPU could result in random reboots.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5