How to track down offending process that is hogging disk activity?

Once in a while my server grinds to a halt due to excessive disk activity. Logging in is hard enough, but running any diagnostics like top etc are nearly impossible. I did manage to run sar, and discovered that the disk activity was extremely high with 100% utilization. Rebooting solved the problem, but I am sure this will happen again. There doesn't seem to be a pattern on when this happens. Last time it was on a Tuesday around 8am. This time it was on a Sunday around 9am. Any suggestions on how I can find out which process is creating this high disk activity? This is on a Slackware server.