Hi, my Imac 27" with external monitor has a weird graphic problem. On restart, it will randomly flash lines across the screen but this problem goes away after a few days of being left on. Once the problem stops, it's ok until restart. It doesn't seem to be a heat or vibration issue, but why does it only happen on a fresh (re)start? The temps don't change in this case. Could it possibly be a Snow Leopard software issue? The monitor is ok with another source.