Pull it apart. Your freezing is caused by the GPU atheon x1600 solder.If you are able to flip the mother board apply a soldering iron to the contacts on the other side of the 1600 chip. (60 sec. per contact) This will cause them to re-connect properly. Some people have applied butane flame (crack pipe lighters) to the top of the X1600's and other graphic processors in iMac's with some success. Thermal paste is important when re-assembly happens.