DFS Targets randomly stop responding

We're having an issue with our DFS targets where they randomly stop responding to clients.  We're using offline files and folder redirection for our clients so when DFS stops working, everyone's computers just lock up.

Here's what we're using:  
Windows 2003 R2 w/SP2 Domain controllers at all sites (all remote DCs are global catalogs)
2x Windows 2003 R2 w/SP2 Domain controllers, one holds all roles for AD, other is not a global catalog
Windows 2008 SP1 x64 file server (DFS Root Target lives here)
We have another root target at our DR site which is connected to the corporate site using a 1GB fiber link
The remote sites replicate userdata back to the corporate office for both backup services using DFSr
The sites are connected using IPSec Tunnels using Juniper SSG series firewalls

Usually rebooting the file server fixes the issue.  All of the DFS health checks come back clean.  All of the dcdiag tests on the DC's come back clean.  I've tried updating the servers to HP's latest PSP and firmware.  They have all the applicable windows updates.  This issue has been happening for almost six months now.  It usually happened about once a month, but recently it's been happening almost once a week.  There are no errors logged in the event logs on the servers.

Most of the time the Vista PCs that use the new version of SMB work when the XP machines do not.  

Any suggestions to fix this issue would be greatly appreciated.

Also, what are your thoughts on using a Windows 2008 x64 file server with Windows 2003 DCs and XP clients?

Thanks in advance for your help.