It's not a cooling problem, it's the fact that it hasn't been properly fixed (sorry if I sound harsh, but it's the truth). The only part of Lawdawg's method, by the sound of your description, that actually fixed the console was when you overheated it. This will get it working, but not for very long. I did the same thing to one of my 360s a while back to fix it, and it did, for a week (IMG:
style_emoticons/default/grr.gif) . Any form of overheating fix (towel trick, unplug the fans, heat gun, etc.) are only temporary, any 360 that's been fixed with any of these methods WILL break again. Don't try overheating it again because the more you do it, the harder it makes the console to fix the right way. The only reason it boots up at all is that one of the solder connections on the GPU is barely making contact, but as soon as it heats up at all, it slightly warps the motherboard and breaks the connection again. I suggest you try adjusting how tight the bolts are on the GPU. Other than that, you could try the 12 cent fix or pay for someone to reflow the GPU.