0.8.7i - Slow memory decrease over time

[Frothingdog]

I recently setup a new Server to run Cacti, Nagios, IPPLAN and Our internal DNS.

The server is a:
HP DL360 G6
running CentOS-6.2
setup with RAID 1 (2 x 250GB drives)
4GB RAM
Linux 2.6.32-220.13.1.el6.i686 #1 SMP Tue Apr 17 22:09:08 BST 2012 i686 i686 i386 GNU/Linux
Cacti=0.8.7i (Weathermap, nectar, thold, monitor, clog)
Nagios=3.3.1
IPPLAN=4.92b
BIND 9.7.3
Webmin 1.580


I've noticed a slow loss of available memory on the system until this morning when I finally got a threshold warning.

Was wondering if anyone has seen this? Or can at least explain it or offer other info.