Overclocking ATI Radeon 5770?

I have an ati radeon 5770 video card with ati catalyst installed on my computer. i ran the auto-tune feature and it set the over clocking setting preety high, but i figured hey its auto-tune so it'll be fine. Afraid not, when i try to play almost any game it freezes and locks up my whole computer making me restart it, in most games the screen shows brown horizontal bars and quits working. I can turn the settings down however and the problem doesnt happen, am i pushing my video card 2 hard or wat? cause im just using the settings the auto-tune reccomended.