I would recommend that you download the app called MacTracker from your App Store. I had a similar issues with my Macbook Pro 13" Mid-2010. Depending on the year, model, and OS that you have it may only be able to support 8GB of ram. With that App it should tell you what your specific computer can handle, I know for me when I tried doing 16GB in my Macbook it did misbehave but finding out that my model is only compatible with 16GB if I make the upgrade to Mountain Lion (which I did). Hope this helps! Andrei.