There are a lot of possibilities , but please give us more info on hardware/os/software.
Until then,
First Try moving your ram around to the other slots and see if the error changes. If so, replace your ram.
If that didn't make a difference then try turning External Cache off in the Bios. If resolved, replace your external cache (or the MB)
If no luck there, try turning Internal Cache off in the Bios. If you don't have this option then replace/swap the CPU.
This can sometimes be a result of heat problems, so make sure you're running at a good temp. Heat causes all sorts of troubles that can make troubleshooting a real pain.