SCO Inode 100% used problem

We have an interestin problem with a SCO Openserver 5.5 box. We have a partition that hold a lot of data in several million very small files,uses about 54% of the partition disk space and 19% of the inodes. We tried mounting a Windows2000 NAS box (Dell 725N) running NFS services on the SCO box to copy these files, but started to experience problems - firstly that the SCO box spontaneously rebooted with the NAS attached, but now every now and again, and it can be weeks apart, or could even be twice on the same day, we get the inode usage shoot up to 100%, some 13 Million inodes dissapear almost in an instant. We haven't seen any gradual build-up, and a reboot brings it back to 19%. There's nothing in any of the logs, and nothing that uses that partition that creates any temporary files, nothing that we can think of. We suspect some form of corruption on the FS, but are having trouble unmounting it and reformatting that partition. Scoadmin not being too helpful.