Q: Colored dots on iMac screen and lock ups

Hi, I am using an iMac 27" from Mid 2010. For the last year there has been these colored dots that would appear over the screen sometimes and every once in a while it would lock up. Yesterday I tried to fix the problems it was having by wiping the hard drive and installing yosemite. Since, the problem has gotten a lot worse and now theres constantly dots everywhere that change when you do almost anything. Here's a screenshot...

It usually looks worse than it does in that pic. It also locks up pretty much as soon as I try to open a program. I tried everything like repairing the drive and smacking the computer a few times, all with no luck. Actually right now as I'm typing this I noticed that the dots are changing every time I type something and when the cursor blinks, maybe this means something. Any ideas? Please help! Thanks