Additional troubleshooting queries:

http://troubleshootingsql.com/2010/02/16/how-to-find-who-is-usingeating-up-the-virtual-address-space-on-your-sql-server/

If the above doesn't help, then a more invasive way to identify the culprit would be to capture XPerf traces: http://troubleshootingsql.com/2011/05/31/tools-tips-and-tricks-12-xperf-memory-usage-and-much-more/

Or capture a Debug Diag...

This is definitely a MTL fragmentation issue. Finding out the culprit is going to be difficult without tracking it. If possible, run the query to gather the largest free block...

You seem to have sufficient available physical memory when the error was reported. Also, the error message reports that you are failing to acquire 576KB of contiguous memory. This might...

I know I asked this on Twitter as well... Check if the plan changes after updating stats with a fullscan on the table.

It would be a good idea to find out what the actual and estimated row differences look like in the execution plan. Since there is a table variable being used,...

Along with the output of traceflag 1222 (if on SQL Server 2005 or higher), you can capture a profiler trace with deadlock events and statement level events to get the...

You need to find out the oldest transaction sequence number associated with a version store. The version store is maintained till no associated open trans are attached to it. And...

Agree with SQLGuru here. You should disable LPIM privilege for the service accounts till you have managed to profile the memory usage for the SQL instances on the box and...

As I said, set MAX SERVER MEMORY on all the instances. It looks like you are experiencing paging on the SQL Server instances. But that is a different topic altogether....

Set Max Server Memory for all the instances in the following way:

Max Server Memory of Instance1 + Max Server Memory of Instance 2 + Max Server Memory of Instance 3...

This is probably because the BPool is not able to grow during SQL startup. If you have multiple instances of SQL on the box, then make sure that the max...

You are on a pretty high build of SQL Server (Cumulative Update package 6 for SQL Server 2008 R2). I just checked the function call on which you got an...

The quickest I would say is to open a support case with Microsoft SQL CSS. If you can reproduce this issue at will, then I would definitely recommend going the...

Could you post which SQL Server build you are on?

Those are pretty old builds. At this point, I am not sure if it was actually a split brain or not. But after you update to the latest CU, enable...