NW6 56% cachebuffer system with 3gig ram

When originally installed last summer, this system reported 72% cache buffers. Currently it is dipping into the mid to low 50s. Not too much loaded, SP2, base NW6 services, BackupExec v9, plus NDPS. No native file access, no iprint, no ifolder etc. I have searched the Knowledgebase for memory leak TIDS, and have updated the TSA to beta 11, latest NSS and NDPS patches.

Files system is NSS with 2 pools, total storage is 70g with about 25g of files. 15 active connections.

When I reboot the server, it starts at 56% cache buffer and in 30 minutes or so goes up to 70%, then sometime during the day that decreases again to 56%.

Any ideas? Seems I should have more available with this much memory installed.

Thanks -David