So my xbox that was crapping out every 3 minutes now runs like a champ after heatgunning it. Only time will tell how long it holds up.
Here's a few tips from my experience that might help out.
1. A wagner heatgun goes for about $25 at your local hardware store, and you'll find them in the paint section.
2. Anti-static bags work well as isolation against heat... The plastic only melted on the reverse side where the heat was applied, everywhere else the plastic was still intact, and didn't melt or shrink.
3. Electrical tape is your friend. Make sure to use it to mask off the isolation layer and the foil layer, and you can use it on top of the foil layer and it won't damage your board when it melts.
4. I didn't heatgun my board exactly how long to wilhelm's estimated time, and again only time will tell if my job holds up, but I noticed that a good guestimation of when you should stop, is when you start smelling the board start cooking, and when the electrical tape starts to melt a little. For me this happened around the 2 minute mark after the 1 minute warm up.
5. My heatgun was about 2 fists distance from the board itself while I was heating it.
6. Using a IR thermometer the board cooled off in a period of 10 minutes, I gave it an extra 10 to be safe before I handled the board and did the opposite side, then reassembled the xbox.
7. Tightening the heatsinks, you don't need that much pressure, use your fingertips on the screwdriver when tightening the screws and when the screwdriver wont turn anymore, you should be good. No need to torque the screws down, by holding the screwdriver in the palm of your hand.