HELP: 7.1 DB Engine performance (not an easy one)

Hi there. We have recently moved from Online 5.02 to 7.1 on an SCO OSR5 platform. Everything works beautifully for a few hours, an then, suddenly, for no apparent reason, the DB engine slows down dramatically. By "dramatically," I mean the new performance is less than 10% of the old one. Once it slows down, there is no way to recover. Even with all users disconnected, a single DBAccess session takes 5 seconds to connect. Everything still works fine, but it takes 10 times longer to do anything. Taking it offline solves it, but only for a few hours. I cannot find a pattern of events that triggers the slowdown. I cannot even find what is slowing it down. All unix monitors report that nothing is happening. The CPU is between 99 and 100% idle. There is almost no asynchronous I/O activity. There is free RAM and there is no swapping. On a disk intensive query, the hard disk light flashes every 10 seconds. onstat does not show anything strange. The user threads are not waiting for anything. Only a few buffers are in use (out of 5000). Any ideas, anyone? Any help is greatly appreciated. Bashar Chalabi Card Tech Ltd