Linked server drivers use the MemToLeave area. I've had a case of a buggly driver leaking memory in MemToLeave and causing slow backups and eventually a system crash.
The thread stacks are also in the MemtoLeave area.
Are you running x86, x64 or IA64 architecture? What drivers do those two linked servers use?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability