Connection to File/Print Server Lost over WAN

I'm running into a problem where file and print connections to our
server get locked up. Our Windows 2003 File/Print server is inside the
LAN of our main site. There are 3 other sites that connect to us via
WAN over MPLS and an IP VPN tunnel from cisco router to router. Every
user at the remote sites uses a Windows XP workstation to connect to
their own shared folder on the server over the WAN. While this mapped
drive appears to be valid, clicking on it slows the computer to a crawl
while Windows attempts to connect to the resource. After a while
Windows gives up and gives an error message stating that the device is
no longer available. At the same time a similar problem occurs with
the mapped printers on the server. Users attempting to print do not
receive any error message at all, but the job never reaches the
printer. The File/Print Server reports an error in the event logs
stating that a connection was established but no data was sent. The
weird part is that I am able to ping the server just fine via IP
address and domain name. Additionally, file and print sharing continue
to work just fine in the LAN where the File/Print server is. The
problem only exists when going over the WAN. To work around this I
have to reboot the server, which gives me another 1 or 2 days before
the problem happens again. I have not been able to pin-point the cause
of this problem despite hours of troubleshooting and various calls to
Cisco.

Any assistance that anyone can provide would be greatly appreciated.

Thanks,
Luis