Strange reboots with new vid card

I just got a powercolor 9800 pro a week ago. So course I gotta O/C it. I run the find max core and mem speeds with ati tool. I am running a glacialtech northpole 1000 cooler on the gpu and copper ram sinks on most of the ram. anyways best overclock I can get is i think 405/360 +/- I cant remember exactly. If I go a little higher sometimes my computer freezes. I go to xt speeds it will run for a while then reboot on its own. I am wondering if it is not a power issue. My card has not volt mods... but probably will soon. And has a 350 core.
Could have something to do with it burning in? Its got the 2.5ns samsung ram it think. it says samsung XXXX-2a.
Any ideas as to why this might be happening would be appreciated.

Some cards are just not good ocers, first put ramsinks on all ram chips because only one ram chip not sinked will cause artifacts, and make sure that the cooler is making proper contact.