We're seeing something very strange on our SQL Server 2005 9.0.2047 DB
on Windows Server 2003 x64 Standard:

1. Most open connections to the database are being spontaneously
terminated by the server. The application log always has this error
message in it:

A user request from the session with SPID 56 generated a fatal
exception. SQL Server is terminating this session. Contact Product
Support Services with the dump produced in the log directory.

But the sessions that are being killed aren't doing anything - they're
mgmt studio sessions, just sitting there, not even running a query.

2. Connection closings appear to be correlated with the "sa" user
running a dbcc command on a large table in one of our two largest
databases; sometimes it runs this:

select (sum(page_count)/128.0) from sys.dm_db_index_physical_stats(8,
1605580758, NULL, NULL, 'DETAILED')

other times it runs a query to check fragmentation. Both large
databases have auto-create and update of stats disabled.

3. Someone has recently started using SSAS on this machine, doing
large-scale analyses of our largest database.

4. We also started getting this error recently:

The configuration information of the performance library "C:\\WINDOWS
\system32\sqlctr90.dll" for the "MSSQLSERVER" service does not match
the trusted performance library information stored in the registry.
The functions in this library will not be treated as trusted.

which I admit is pretty obscure but I thought I'd throw it out there
anyway.

I have no idea what's going on, so any theories are appreciated.

Thanks,
Seth