Maybe something shifted in the case when it was moved or it was banged on something. I had this happen a few times and usually its a ram stick or maybe your video card moved 'O so slightly but not enough to notice. If you know what you are doing, then remove and put the ram back and see if that helps. If not then try the video card. Check the cables and see if everything is fine. This is a normal thing and I find this happens quit often being a computer repair man. I fix many computers that I only spend 2 minutes on and which is awesome as its easy money.