Q: Computer tells me to restart.

Sounds like a kernel panic. These are usually hardware relared, often caused by faulty (or loose-fitting) RAM.

What is a Kernel Panic?

http://support.apple.com/kb/TS3742

and  http://support.apple.com/kb/TS1892?viewlocale=en_US

http://developer.apple.com/mac/library/technotes/tn2002/tn2063.html

More details here:

http://www.thexlab.com/faqs/kernelpanics.html

Useful articles on how to avoid and eliminate kernel panics here:

http://reviews.cnet.com/8301-13727_7-10344626-263.html?tag=mfiredir

http://www.macworld.com/article/2027201/how-to-troubleshoot-a-kernel-panic.html# tk.nl_mwhelp

If you have more than one user account, you must be logged in as an administrator to carry out these instructions.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Console in the icon grid.

Select the most recent panic log under the heading System Diagnostic Reports on the left. If you don't see that heading, select  

View ▹ Show Log List
   
from the menu bar. Post the entire contents of the panic report — the text, please, not a screenshot. In the interest of privacy, I suggest you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header and body of the report, if it’s present (it may not be.) Please don't post shutdownStall, spin, or hang reports.