North Bridge heat storm causes ~

I start with 4870, the North Bridge heat storm, in the hands of North Bridge heat sink can not keep more than 5 seconds! When used in the original 7300GT is not so hot! CPU overclocking now and can not, 7300GT time, CPU has been 3.2 to the ultra-stability, overclocking is now able to hear through the self-Di's voice, but then restart immediately.What I can not find any reason to hope that expert advice!

in adding a North Bridge fan, Express about 3 to 5, is not enough, another North Bridge direct radiator, 40 to 60 or so can, and should pay attention to is the initiative of the heat.Enter the BIOS is also a look at the graphics CPU temperature and the temperature, what brand of graphics cards?
Board what?
Also Finally, there is a lack of power of your power, try to change the 400W rated.Question of what came to me.