I'm going to post a comment i made in another thread, it has to do with kingston ram;
I had a similar problem with RAM i purchased made by Kingston, for my first generation Mac Pro. I bought 2x2gb sticks. I already had 4x512mb, so I would have had a total of 6gb. I installed the ram and it came out at 4gb total. I repositioned the ram and restarted the mac, and got my full 6gb.
After about 2 weeks, after doing some random fiddling around with settings and such, I checked my mac pro's system specs (just to drool at its awesomeness) and noticed my RAM went down from 6gb to 4gb. I checked everything; repositioning of ram, swapping ram out (isolated old ram, isolated new ram), ram tests, pram resets, etc. What I found that the kingston modules were showing up as 2x1gb. So i tried running them on their own (only one module in the mac). Sure enough, a red light flashed on the riser card and one of them had dropped down to 1gb of available memory, the other was fine. When paired, I got a total of 2gb. I got it quickly replaced for new working ram. I'm still a little skeptical of kingston, eventhough its brand name. Hopefully the new modules I got will last me a good while.
Here's my thread if you wanna read more into it;
https://forums.macrumors.com/threads/791432/