The display boots up with vertical yellow lines.

It started out booting up with yellow vertical lines and grey dots in the background and that was all you could see. When I boot into safe mode the lines are still there (sometimes) but at least I can see everything and get into what I want such as system preference in order to change the display settings. The first five times I fixed this by booting into safe mode and changing the display resolution from the default to something else. Then the Mac would boot up just fine and last for about two weeks at which time it seems to reset its display resolution to the default setting and therefore revert back to this display problem. Today, the problem arose again but this time when I change the display settings and restart, the settings do not stick. It automatically resets to default and the problem still occuring. I have tried an external monitor and the problem still exists which tells me it's not a LCD problem. I'm wondering if it could be the pram battery since the settings aren't holding. If not, I’m thinking it’s a bad video card. Either case, I'd like to try and fix it and get some more experience with Macs under my belt. I’m a PC girl and I only starting learning Macs this last year. Suggestions are greatly appreciated. Thank You!