For some time now, my MacPro has seemed sluggish and slow. Running Tech Tools Pro, I noticed that it listed 2Go of RAM...but I installed 10Go RAM when I purchased the unit, five years ago. What could explain the missing RAM?
Remove all the DIMMs and riser boards, clean all the connections with a pink pencil eraser and rubbing alcohol, reassemble and test. You might have to reseat the riser boards a few times - I have had to on mine in the past.