Odd. I had this problem in 10.6 (Snow Leopard) and it went away when I upgraded to 10.7 (Lion). The most obvious symptom is the machine does not seem to be able to make use of the inactive memory (the blue slice of the pie in Activity Monitor). I did not notice the problem at all when I first started using 10.6. When I hunted around on the help forums a few months ago I found lots of users with the symptoms, but nobody acknowledging the "validity" of the problem. In fact a lot of deniers out there. I wonder if it was quietly fixed or will recur after the system gets used for a while.