Win2k Server rebooting

I have a Win2k Server with 2 5-drive SCSI array’s on Adaptec ATA 2940 SCSI controllers (firmware has been updated). This server had redundant (hot swappable) 300 watt power supplies. It originally had one 5-drive array and was running fine until we put in the second array. Once the second array was installed, the server started losing power when we ran a backup or tried to move files from one parition to another partition on the same array. We upgraded the power supplies to 400’s. Now thething is not losing power but it is rebooting when when we try to do anything such as run a backup or move files. This Server is our primary domain controller as well as our Exchange 2000 Server. Any ideas? Thanks.