I have recently gone from 6.5 SP3 to 6.5 SP4a, and now I have all the
symptoms described in TID 10099199, viz., any attempt by an NFS client
to read an exported fs causes the server to disconnect any Windows
users (as well as the Unix client). Drive mappings disappear and cannot
be restored. Rebooting the server seems to be the only fix, but it
stays fixed only until an NFS client connects.

Alas, TID 10099199 says all this is fixed in SP4, but for me the problem
began when I installed SP4a.