I'm having the same problem. If I leave it alone on the table it works fine but when I lightly move my laptop around, the beach ball will spin continuously causing me to restart my laptop with the flashing question mark. What I did next was I opened my laptop up and played around with the flex cable and tried to restart it again. The laptop works after I dissected it but goes back to the beach ball spinning/flashing question mark if I shift it (i.e. leaving for school and putting the laptop in my backpack and opening it up again in class).
What I don't understand is that I had the flex cable replaced a few months back through Apple because I was having this same problem back then and now my hard drive is acting up again. I'm not under warranty anymore so what should I do? I know it has to be the loose connection between the HD and logicboard but I am lost. I'm using the broken laptop right now but it will probably act up again if I shift it around.
Note: I've never done any major damage to my laptop nor dropped it.