Alright, I just don't know what to do anymore... I had upgraded my late 2008 unibody MacBook Pro to a WD 5400 RPM 500 gb drive. Started getting beach balls randomly (try to open a file, beach ball, listening to a song, beach ball, couldn't make it through a movie without it stalling with a beach ball 4-5 times). Reformatted, clean snow leopard. Same. Reformatted, clean leopard. Same. Cleared the cache, fixed permissions, rebuilt spotlight. Nothing. Reinstalled an old 160 gb drive with snow leopard. Zero beach balls. Worked perfect for a month. Thought it must have just been the drive. Just bought a new 640 gb western digital, reinstalled snow leopard. EXACT. SAME. THING. Beachballs doing random things (or nothing). What is going on here??? Help! Anyone have any solutions? I have all os x SL updates installed. There was a "performance update" for intermittent hd stalls for 10.6.1 but you cannot install it under 10.6.2 (presumably it is included).