Mine runs rock stable at 450MHZ 2.3V. Change to 2.4v and see if it runs stable. I mean run starcraft with all the cheats and 8 players. If it does not bounce you out of the game after 1 hour, it is stable. What kind of mobo do you have? Also get alpha cooler from www.2cooltek.com One guys told me that there was a 17C' improvement! I am getting it in 2 days.
I have a K62 400 that is clocked up to 450. The motherboard is well cheap and only goes to 450. The voltage has to be at 2.4 to make it stable. I am using a very cheap standard+heatsink fan. It could definately do with some more cooling as it is starting to smell a bit burnt but otherwise running fine with no lock ups.