Tom,
You and I seem to struggle with similar issues. My Vista machine is much more stable with projects from 10 since I took these steps:
1) AC 11 updated to 1040 and libraries updated.
2) Latest driver for nVidia card.
3) Upgrade to latest DirectX driver from Microsoft (9c).
4) When opening 10 project, load only AC Subset Library 10. Do not load libraries from 10 or earlier.
Hope that helps,
Pete Read
ArchiCAD 12; Artlantis Studio 2
MacBook Pro 2.4 Core2Duo, 2GB, OSX(10.5) and XPpro(SP3)