Did you try to stop/start the NFS Demon service on Windows? Maybe it's "locking" the connection for just those 2 hosts.

You can also check on the Sun boxes, if there is a /var/lib/nfs/rmtab (or something similar) which you might want to empty or delete entries for the 2 specific Sun boxes. Maybe there is some similar file on the Windows server which you can edit.

You can also check:
Linux NFS faq

If this doesn't help and you already tested a lot you might think of not using NFS (I don't like it, too many problems on some machines) maybe try it with normal windows shares (SMB) and install/use a Samba client on the Sun boxes?