The issue you are experiencing could be related to the memory sticks you are using. Ensure they are fully compatible with the memory controller on the processor running at 1.5V.
Test the system with one memory module at the time.
I don't have them here right now, just remeber them as Kingston ValueRam DDR1333 specified (according to SPD) for 1.5V.
The question is, whom do I trust the BIOS, that says 1.500V with default settings or IDU that reports 1.57/1.58V.
Also, if I manually set the memory voltage in the BIOS to 1.500V, IDU also reports 1.508V, and all memtetsts I did (running > 24h) are OK.
So again, whom do I trust?