Systems Lock up overnight.

We have 30 PC’s in the office. 28 new Dell Dimension’s running Win98 and 2 Gateway 233’s running Win95. These systems are connected by a SCO UNIX Server. Over the past 3 weeks 8 of these systems lock up overnight. Seven of them Dell’s and One ofthe Gateways. Not all 8 lock up every night; some time it is 4, 5, 6, or 7 of them. Power Management has been deleted in Windows and Disabled in the Bios in all machines. They have been disconnected from the network and continue to lock up overnight?