blinking gray lines/bar on screen

Hi!

A few hours ago my MBP mid 2012 13" (non retina) crashed. I have the latest yosemite 10.10.4 and using a secondary display. Bought it June this year and protected it with kuhvuh plastic casing/covers.

Anyway, so I continued using it after rebooting. A few minutes later i saw blinking lines/bars on my mbp screen. it doesnt appear on my secondary monitor. When I tried moving some windows on my mbp screen, part of the lines disappear (The lines that are covered/touched by the moved window.) I made a quick search on the problem but most of the articles I see are for mbp 2009 models. I can't find anything for MBP 2012s.

The lines/bars were quite thick and appear across the screen. It is not continuous. More like dashes but thick. They look something like this:

|||||||||||||||||| |||||||||||||||||| |||||||||||||||||| |||||||||||||||||| |||||||||||||||||| |||||||||||||||||| ||||||||||||||||||

They were blinking/flashing, and gray in color. It went away after moving the window all over the screen.

After an hour they re appeared. but this time only lasts for 1-2 seconds. And then disappears and reappears. It only stopped when I moved the windows on my screen.

I have experienced several crashes before this, but this was the first time that these lines appeared. Most crashes happen when I have several of apps open. As well as multiple tabs on firefox. It doesn't crash on a daily basis.

I was wondering if this was an overheating problem, or something else. Hope you guys can help me.

Thanks!