samba - Oct 2012 - Users randomly unable to login after machine left idle on boot

Everyone,

 Perhaps this is an known issue, but:

For some time, we have inconsistently been running into an issue where end
users are unable to login to their machines if they have been left booted,
but idle and not logged in, for some amount of time (it is not clear what
the minimum is) after being booted. In (almost) every case, rebooting the
machine restores their access. Caveat this only because internal IT staff
have only recently decided this is a priority to resolve, instead of a
lower priority irritant, so I don't have anything more substantive than a
bunch of inconsistent anecdotes to base a problem description off of, and
there's an outside chance that we're looking at multiple issues or two
inter-related problems. Unfortunately, the problem is not reproducible,
either... machines / users that had it happen semi-frequently no longer run
into it, other recent reports come from machines / users who have not run
into it before. One tech did get wireshark on it at one point, and found
requests going out, but replies not making it back in again (this is being
relayed through a second party).

 There was a major instance of this last week, which resulted in a number
of users being unable to login, and the resolution was kind of messy, with
the result that the problem got pushed up the stack. We are going to
attempt to replicate that instance on Monday, in hopes of getting multiple
machines in the failed state so that my partner and I can do forensics on
the resulting logs and network captures... but it never hurts to simply ask
the experts if they've run into this before.

We're running samba 3.4.7 on Ubuntu 10.04 (Lucid). The clients are all
Windows XP SP3. Fast Ethernet. Local profiles, centralized network
authentication via Samba/LDAP. Samba server sits on a different subnet than
all the clients.

 I'm thinking that I'm running into a network timeout issue, where a
connection is closed but not re-opened, after some period of idleness. Most
likely on the client, but I don't know enough to be sure, or where to look
in Windows or Ubuntu.

 Does this problem description match any known issue? I've already trolled
through Google extensively, and haven't found an exact match, or at least
not one with a solution attached.

Thomas