The only weird consistent thing is that it only happens with a 2000 server doing the file serving, never when linux is doing the serving.

I'm not an expert, but this should be related to the fact that Linux uses the more flexible POSIX type file locks while your w2k servers use the more restrictive win32 file locking (no overlapping ranges, no range splitting or merging, and no upgrade or downgrade).

EDIT: typo