There’s no such thing as a 100% stable system, but I suggest first:
run in 64 bit mode and replace as many plugins with 64 bit versions as you can
increase your RAM - 8 GB on your desktop and 4 GB on your laptop won’t cut it. Get at least 16 GB, and running in 64 bit mode will use that ram.
move Cubase and Nuendo preferences out of the Preferences folder and start fresh.
make sure you have latest versions of all plugins
make sure you are using latest audio drivers for your interface and drivers for the intensity card.
Having said that, I’ve seen older Mac towers that we could just never get running well, even after a clean install. Could be things like old video cards, bad firewire bus, who knows. In all cases upgrading to a newer faster tower solved the issues.
The new Mac Pro’s are awesome, but I haven’t yet tested Nuendo or Cubase with them. I have a 12-core on order for one of my users, so should know in a few weeks.