Didn't fix it? Well, if the problem is not overheating, it could indeed be caused by cold solder joints, cracking, voids, etc. A potentially simple (with proper equipment!) way to fix it is called reflowing the processor/board. This process entails heating the board from underneath, while applying hot air or infrared heat from a 'rework station' to the processor until it reaches/exceeds the melting temp of the lead free solder, approx 213 degrees celsius. Upon reaching the melting point where the solder should 're-flow' back into it's proper shape and position, the temp is dropped and the unit is allowed to cool undisturbed so as to allow the solder to re-set properly. I have seen videos of individual attempting this by removing the fan and air shroud and letting the box overheat (I suspect they are simply reproducing the effects of the towel trick), or others that place a metal container on the offending chip and actually burn material in it, so that the heat is transferred through the metal, to the chip, and hopefully to the solder. Others try to duplicate the effect with an air gun. The main problem with all these methods are you have no way of effectively controlling the temp unless you use a rework station (see video http://www.youtube.com/watch?v=ndeq6cAA8MY
to get visual on what I am talking about, pretty sure the electric griddle is not a professional aspect, but saves on the cost of an actual board oven). I've found auctions on Ebay for a guy that will do this properly for $40 ($30 to repair, $10 to ship your Xbox back to you). You have to ship it to him in first place, so count on about $50 for this total.