dont use the heatgun..redo the xclamps and then remove the case fan..put the board back in and screw it up to keep it flat..hook the dvd drive back up and then plug the fan back in but leave the fan out of the case..then rest it on top of the dvd drive..half on the dvd drive and half on the cpu heatsink (tall one).
then turn on and bake for 15 mins..
(IMG:
http://www.llamma.com/xbox360/repair/images/x-clamp/xclamp%20074.jpg)
the heat safety cut off is actualy under the cpu not the gpu..so keeping the cpu cool will stop it getting the 2 red lights overheat..2 red lights actualy cuts power to the gpu/cpu so will not overheat..
so green lights is good..1 red light is good and 3 red lights is good..if ya get 2 red lights it isnt working..
once you have baked for at least 15 mins let it cool down for a further 15 mins and then test it with everything back in place eg the fan hood and fan..
it could also be that the bolts arent tight enough..make sure they are short enough or have edequate washers and arent touching the top of the gpu..also if you are using your own hardware and not apurchased kit ya may need to modify things slightly..if you are using 2 metal washers they may be ever so slightly too high to give adequate pressure to the chip..in these cases i use a shrimm made up of flat tin foil folded 3 times..but this MUST be cut to fit the 2 small dies on top of the chip. tin foil conducts electricity so you dont want it touching anything it shouldnt
i actualy use my own method for red ring repair which is very succesful but doesnt require any bolts/washers etc and retains most of the M$ hardware but i wont go into that as the bolt/washer method is fine if done correctly..
avoid the heatgun at all costs if you dont know what you are doing..in some cases (for me elites with e74) the heatgun is the best option but if not done long enough its useless and if done too long your xbox is dead forever. so use as a last resort..the bake method above for 15 mins gives the right amount of heat and is far safer than heagunning..
sorry this is off topic but i couldnt resist.
thought i would try hesters suggestions first as i have no experience with a heat gun