There have been a few more reports of lag with the TPU server.
I've performed some diagnostics and haven't found anything conclusive, except that the uplink buffer size for my ISP is excessively large. I don't think that's the problem, though.
I did end up making some changes:
 Updated the TPU server's NIC drivers.
 Disabled the Windows XP firewall on the TPU server.
 Ran the Speedguide.net TCP Optimizer on the TPU server, which made a lot of changes.
Let me know if this makes any difference.