Stop cached folders off line

I have redirected the My documents folder of all my laptop clients (XP
SP2) to a share on 2003 Server. These folders are set up to be
available offline.

When my laptop users leave the office for longer than a day, the cached
copy of the My documents folder disappears. They then cannot see any
data. They do connect to the office network over a normal VPN and
the folder does try to sync but gets cancelled or fails due to the
link. When they get back the office, it syncs up again fine. Does
anyone know why the cached copy would disappear? Does it have anything
to do with a failed synchronisation attempt?
Also, can anyone tell me a recommended setting for the "configure slow
connections" GPO setting for offline files. I would like it to sync at
anything higher than 10mbps to accommodate our wireless network but rule
out any WAN connection.

Thanks in advance!
KoySys