Very Curious ???????

I noticed that from time to time one of my databases ( I use NT 4 SP 6 with 5 instance running on it Oracle server ver 8.0.4 ) keeps the server's hard disk working like hell even if there is no one connected to any other instance at all.

Could be this from the SMON ( defragmenting for instance ) , it does not return any error messages but it keeps working like this for 2 or 3 days continously then it gets back to its normal status.

Also performance degradation happens during this process.

What could be the cause ??

Thank You

Have you checked Memory usage?
Maybe it's beyond physical memory so the disk activity is swapping/paging.
Another thing: I don't know if in 8.0.4 you can, but try to create your TEMPORARY tablespace with the TEMPORARY clause, not PERMANENT.