A couple days ago while playing a Mac game via Steam, my screen froze. I was able to quit the game but the screen had numerous thick vertical vertical lines on it. I restarted the computer and it again froze but this time was covered in small pixels. Next restart I reset the PRAM & that seemed to resolve the problem. The next day I was playing again & the same thing happened again, froze, vertical lines then pixels. I tried to reset PRAM again but to no avail. Next I reset SMC & this resolved the problem but only temporarily. It later occurred again & neither PRAM or SMC reset would work. This morning, 8-9 hours from last freeze the computer started fine & I was able to write emails, check FB and read the news. I started processing some photos using Photoshop & LightRoom and it wasn't long before the screen froze again and the same screen weirdness appeared. Neither PRAM nor SMC worked today.