It's a known problem on the P5B with quad core. It is FSB limited with quad cores, I saw some reviews where they needed 1.65v for the NB at my FSB. At 389MHz, vNB 1.35 will reboot on windows start while 1.45v is ok. At 400MHz, same vcore, I need vNB 1.55 as it will reboot at 1.45v
With C2D, there's no problem