High RAM memory usage in RHEL 5.7 with no I/O cache and no intensive processes running

Issue

• A RHEL 5.7 system shows RAM being fully used, even though there's almost no disk cache usage, and no intensive processes are being run.
• System performance is generally slow.
• "free" output shows little to no diskcache & buffer usage, as opposed to https://access.redhat.com/knowledge/solutions/1138