Lion was absolutely broken in terms of memory management from 10.7.0 through at least 10.7.3sh. (Beach Balls during pageouts, constant memory pressure, weird page-out while using Safari + Aperture - really made me miss 10.6.8) Things got much better in 10.7.4, and 10.7.5 has been so reliable and well balanced that I'm afraid to ever leave it.
I was able to remedy this by using the terminal command "purge" occasionally, and also disabling the dynamic pager[1]
[1]http://workstuff.tumblr.com/post/20464780085/something-is-de...