When MacBook Pro is turned on, the startup chime sounds and there is hard drive activity, but screen remains blank. No video appears on the monitor, even if I trigger F7, +F2, or turn up the brightness. Not a backlight issue because not even a very faint picture appears. I know the screen itself works because this issue happens intermittently (it fixes itself sometimes, and the screen works perfectly). I tried zapping the PRAM and resetting the PMU.