The first thing to determine: is it the monitor or the video card. Swap the monitor with that from another PC and you'll know in an hour or so.
After my computer has been on for about 1 hour it starts getting shimmery wavy horizontal lines on the monitor. I thought maybe it was getting to warm for some reason and took off cover to allow more air but it didn't help. I also blew all the dust out. The video card is less than a year old. Any ideas?