Man , I thought you had this one taken care of.
Have you run any memory tests ?
I think you just have a bad stick of ram, I would hope thats all it is. Unless there is something that is eating your memory. Do you have two sticks of ram, if you do, shut down, unplug, then take one out, and start up.
Before doing this :
Start task manager, and look to see how much ram is being used, compared to what you have.
EDIT : especially if you have ECC . Many people buy this stuff, and it's really only good for servers and such.