I have a very similar issue. I have BU Exec 10d service pack3 with Exchange, remote servers, and DLO options installed on a Win2k3 server backing up one Exchange 2003 server on a w2k3 server. For days it runs flawlessly, then one or two days in a row...
I have an AIT-3 (700v) on BE 10d and see a very similar problem with compression. I get 100G no matter what I do. The drive is brand new, the firmware is up to date. I get compression ratios of 0.93 to 1.01 to 1 conststantly.I am running Server 2003....