Provided it is your Northbridge, the way is the same I followed to fix similar crashes on my mainboard: Remove that cooler (it is usually fixed by means of two push pins. Loosening those without damaging the mainboard usually requires getting the mainboard out of the case up front).
Once the cooler is off, remove the thermal pad from it (or thermal grease, but I expect a pad to be in place). Make sure not to scratch the cooler surface in the process, so use no knives, screwdrivers or the like. Your fingernails are ok though.
Then wipe everything extra-clean with isopropyl alcohol, toluol, or, even better if you can get hold of it, chloroform, and apply a dot of good thermal grease. Thermal grease conducts heat better than those pads, and by selecting a good thermal grease like the Prolimatech PK-1, you can further optimize things. Then put the cooler back in place and fasten it by pushing down the push pins.
If there is enough space above your northbridge cooler (no graphics card getting in the way), you can even replace the cooler with a better model (better = more surface = more ribs or even fingers). Such replacement northbridge coolers are available in the internet for like $12. I could not do it because my graphics card is hovering right above the NB cooler, but replacing the pad with the grease fixed everything for me nicely.