I had limited success using lawdawgs method. The xbox probably worked a couple months afterwards, then rrod again. Just loosening, overheating, and retightening didnt help after that. So, I took a heat gun, and an infrared thermometer and with the heatsink removed, heated up the gpu to 270ish degrees. I did it very slowly, taking care to use a circular pattern . Put the heatsink back on, did the overheating trick, but this time I blew the heatgun directly on the gpu heatsink as it was overheating, and being careful with my thermometer not to get it too hot. After that, it seems fixed again, but who knows for how long.
I think part of the problem with letting the xbox overheat itself, is that it doesnt get the gpu hot enough. It seems as though the cpu overheats first, and throws the overheat lights. So if you have a heatgun, you can help the gpu along, or if you can isolate the fan to only blow on the cpu, you could ensure that only the gpu is overheating.
Also, its a good idea to cut out the metal tray to give clearance to your bolts. If you dont, then when you bolt it all back together, it will put your xbox on a bind, which is probably not good for solder joints either. Also, the thermal pads on the memory may not touch the tray properly if they are spaced more by the bolt heads.
Good luck all