I have 2008 Mac Pro, and RAM sometimes shows up as 6GB and sometimes as 8GB, the little red light on the riser comes up sometimes and sometimes not. The RAM itself its not disabled, it just shows up as 2x1GB instead of 2x2gb. So, all modules *are* detected. Can I only replace one stick, if it needs replacing? or is it something else? I already swapped everything in and out and changed risers/positions.