Just reading this thread. I have a Powermac G5 with a memory controller (Northbridge chip) with this issue. If I heat it up with a heatgun a bit, system boots fine, and will sometimes run for 12-18 hours. I bought a logicboard from epay for cheap, and it works, but has some issues, so I'd like to fix my original board.
When you guys put a piece of solder on top to tell the temperature, are you using lead-free or leaded solder to do so? I assume my 2004-2005 Powermac is ROHS compliant, so would be lead-free? Since lead-free has a higher melting point I would assume that would be what I would want to use so things get up to an appropriate temp?