Problem unknown. Please Help!

I have a macbook pro with OS X 10.9.2, 4gb ram and a few days ago ot started to get really slow and the screen turns gray every 1 minute or 40 seconds. When it does that the dock hides and then reappears. I have tried killing all open apps and many kinds of hard reset. The HD is almost full(2.43gb free). I dont know what to do to fix it, mainly because i dont know what it is. I dont know what cause it I haven't installed new apps or software updates. Thank you for reading. I'll appreciate your help.