If you are running ‘vanilla’ … that is:
1-Cubase 6 (with the VST Collection only ).
2-Mac OS 10.7.2 (Lion) using the built-in audio hardware for
input and output. (MIDI I/O you did not say in this thread)
3-You have no ‘3rd party’ stuff attached (hardware or software)
4-You have tried re-installing Cubase 6.
(also consider re-installing Lion)
5-Check to see if your Ram is still properly ‘seated’.
Sometimes the modules can ‘wiggle loose’
especially on a laptop.
And you are still getting crashes…
At this point I would think about having the motherboard
on the 'puter checked.
Sometimes Apple ships a bad one.
I has happened to me more than once.
Good Luck and
Sending much Aloha