Did you add any packages? Did you make an upgrade of OpenACS? Are you surfing your site with the developer support bar turned on with database support (to see if the problem lies in the database of with the AOLserver)? Does the apache have to run on this machine? What partitions are growing (run "df" every minute and compare the results), which will help you narrow down the problem further.
If you have not done this, setup multiple partitions or even better an LVM to detect where most of the disc activity occurs. Keep the system Logfiles away from the AOLserver binaries away from the actual web directory that hosts OpenACS.
My approach would be to look for errors either within OpenACS (which you can detect if you installed any new packages or made an upgrade just before the performance problems hit or if the developer support bar shows some *really* slow queries in e.g. the "My Workspace" page).
If no errors can be found there, try to narrow it down in the system, maybe something is wrong there as well. Alternatively try to use the latest AOLserver from CVS to see if the problem goes away.
