The best idea I've come up with so far is a pair of memory leaks in Windows - particularly one related to WMI. After a while, the memory allocated to the wmiprvse.exe service will reach 512MB, which is a cap - at this point any remote WMI calls will fail. After a few seconds, some garbage collection will occur to free some memory, and SQL Monitor will connect in again.
I've scheduled a hotfix to be applied, but my OS team is slow to roll these things out, so i can't state if this will definitely solve the problem...
For ref, the Windows 2008r2 hotfix is here: http://support.microsoft.com/kb/2832248
, and the vanilla 2008 one is here:http://support.microsoft.com/kb/958124
If that sorts out your issues, please let me know