http://www.atareao.es/ubuntu/conociendo ... ria-cache/
A downloadable Python script freecache.py automatically drops the excess of cache RAM that can freeze your desktop system.
it happens to me when I use Qbittorrent and browsw Flash videos/games or banners with Chromium.
But after browsing I founded a manual solution, that now has been implemented by this blog I do follow.
It is in spanish, but you can use google translator.