Currently Being ModeratedDec 10, 2011 3:45 PM (in response to nOmada)
The gray screen with the restart is called a kernel panic. Kernel panics are usually, but not always, caused by hardware, and mostly by memory issues. First step would be to reseat your memory (pull it out and put it back in).
See this FAQ for other steps: http://www.thexlab.com/faqs/kernelpanics.html