Sometimes server restart automatically to stop automatic restart of server please un-check the marked option in below snap shot. Also try to boot server in safe mode and check for the similar kind of issue? Another useful step is go to run and type msconfig|startup|uncheck startup items which are not required.
Based on the dump file analysis, we can find either system process/file name/ hardware failures along with the respective bug check string codes.
Here is the link for various bug check codes and troubleshooting steps
Some times dump file partially corrupt and which is not allowed to analyse the file to find the root cause of server shutdown.
Below link helps you if you have corrupted dump file: http://blogs.msdn.com/b/dougste/archive/2009/06/12/this-dump-file-is-partially-corrupt.aspx