There are a number of reasons that the SOS Server might hang or crash.
In the SOS Server 3.5.x Server Manager under the Debug tab, please enable verbose output. The next time you get the Dr. Watson error, email a copy of the log to
support@sourcegear.com.
Also, check this KB article to see if it might be relevant to your situation:
http://support.sourcegear.com/viewtopic.php?t=10
If you don't have 6.0d, what version of VSS and the ssapi.dll file is installed on the SOS Server machine?