Your RAM chips are bad, or loose. You can research "checking RAM," and "Changing RAM" to see what to do. I was also a total nerd at doing anything to my Mac, but I went on Other World Computing, found the right RAM, and then looked at the video about how to change it. It was easy, really.
If you are uncomfortable trying to do these things by yourself, then yes, do take it into the Apple Store. They know exactly how to do this in no time. They may charge you for new RAM, but if this makes you nervous, it's worth it. And if anything need tweaking afterward, it's on Apple, not you.
It may be one of the chips came loose, in which case, they can fix it in 5 minutes.