Thanks for your suggestion. I have 2x2gb. I tested them using Memtest86+ @ DDR2-1064 and it passed (~3 hours).

I have been trying to reduce NB and PLL voltages. However, in order to have stable hibernation restoring, this is the voltage it needs. I wonder if there are any other parameters (like CPU/NB GTL Ref Voltage?) can be adjusted to counter the high NB voltage.

The failure mode of after hibernation re-boot looks like this: Right after system restore->Super Pi->Select 1M->Run->Crash and Reboot immediately.

Does this scenario tell me something?