inspiron 531s bad memory slots. HELP

AMD Athalon 64  x2 dual core 4400+ 2.3ghz

32bit Vista

4x1GB 800mhz Ram

new PSU

Through a process of elimination it looks like two of the memory slots are bad on my MB.  I've been running 4GB of 800mhz ram for years but recently i started getting random reboots, lockups or BSOD. I can run any 2 pairs of my ram in the top 2 slots but running with ram in the bottom 2 slots causes freezes and reboots.

BIOS settings I could change had no effect.  Using a PCI video card did nothing to help.  Running Windiag or the dell diagnostic memory tests shows no errors.  Running mini XP works fine but all my diagnostic discs apps(UBCD HIREN etc) find no faults.  Only running Vista Home Premium shows the issue.  I'm guessing there's some issue with Ram access and SATA or duel core that triggers it.  I even installed Vista on a blank drive from the original CD discs and it froze up 5 minutes after booting up.

Other than replacing the MB, are there any options for me?  I'm stable running on 2GB so at least now i can backup the drive and even consider a bios upgrade. :)