I had this after going from Lion to Mtn Lion. Was very buggy, slow, and crashy.
Just did a fresh install last weekend, and ML is now running like a dream. Did a clean install and copied files manually from backup image... No migration assistant and old pref files, etc.
Took a heck of a lot of work to get everything back to how I wanted it, but the clean install made my machine run like I expected it should!!