I had a similar problem on 10.6.8 in June when the 8GB of RAM in two of my MacBook Pros filled up to 100% over an hour or so while presenting videos. Using Activity Monitor, local Apple support suspected third party software to be the cause of the problem and recommended both computers be wiped clean, which I agreed to. Two months on, the problem has not returned. I moved to Mountain Lion about two weeks ago. Hope this helps.
Robert Hicks
Melbourne, Australia