mapped dfs drives missing

This is a really odd problem. We just implemented dfs. Our
company has two drives that need to be mapped. We created
the drive mappings in the logon script and used the dfs
path to the folders. About 90% of the time it works fine.
The other 10%, one of the drives (always "Q") is missing.
If the user logs off and back on the drive is there. This
is not subject to any particular user or workstation. It
has happened to almost all users and on various
workstations. Has anyone ever heard of this?

Here's the setup:
Server: W2003
Workstations: W2k sp4
The logon script is implemented through group policy