There could be a couple of issues.
The simplest is the monitors scan refresh system to see if there is any video input isn't very good or has a very quick give up rate.
It could be how the video card reacts to waking the computer up. Make sure your drivers are up to date. Also check in you sleep settings for windows 7 on how the computer goes to sleep or wakes up.
The motherboard might also have a power saving setting that sometimes messes up the wake up of your computer