well i have the 21.5 inch iMac on OS X Snow Leopard 10.6.4 with 3.06 GHz Intel Core 2 Duo and 4 GB RAM but anyway my problem is that when i play runescape for a while at a random time the screen just flickers and the computer freezes so i have to restart it by pressing the power button off and on i dont know what to do so what should i do??

Hmm is it just runescape? Or is your computer doing that for anything? If it's just runescape, perhaps change the graphics or install the runescape Jagex has on the runescape website. If that doesn't help you then I don't know.
Hope I helped.

Hmm is it just runescape? Or is your computer doing that for anything? If it's just runescape, perhaps change the graphics or install the runescape Jagex has on the runescape website. If that doesn't help you then I don't know.
Hope I helped.

try and change the settings

I have that exact same problem too, playing in resizable mode..... isnt awesome, hope its not a incompatible graphics chip (9400M)