Purple horizontal lines flickering across screen on MacBook Pro?

Okay, first off, the basic information:

MacBook Pro (13-inch, Mid 2012) running OS X Yosemite version 10.10.3

Processor: 2.5 GHz Intel Core i5

Memory: 4GB 1600 MHz DDR3

Graphics: Intel HD Graphics 4000 1024 MB

Now, for the problem. Maybe a week ago, I was scrolling in Safari and Chrome. Somewhat blocky purple lines started crossing the screen just below the middle of the screen. They flickered when I scrolled up or down with the mouse, and in a panic I exited out of both Safari and Chrome. The lines went away then. I freaked out a little, and hoped it was a one-time thing.

Tonight, I was in Safari and the blocky purple lines started doing the same thing, only more pixelated with some green in them. When I went to take a screenshot of the problem, they went away when I moved the... "screenshot box", for lack of a better term, over them. No matter what I do to move the screen back or forth or what-have-you, they don't come back. There isn't any rhyme or reason for when they show up, other than when I'm using Safari.

I ran a hardware test (when you power off, press D while powering back on, and it does the test) tonight after backing up to a hard drive and it came back clear. So... has anyone else experienced this, or does anyone else have an idea of what it could be? No amount of scouring the Internet has turned up anything for me. I'm a broke college student who won't be in America next year, and if my laptop goes caput, I'm kinda done for. I've never dropped my laptop, and it's cushioned when it's in my backpack. It's only overheated twice due to my stupid freshman self putting it on a pillow. When it overheated, it once restarted and once just logged me out of the laptop for a moment. I keep it on a laptop desk now and monitor the temperatures and fan's RPM. It's out of warranty (of course, isn't it always) and I have Apple-certified technicians at my college, but I don't know how much they could help. So... um... help?