iMac 27" 2011 Flickering Screen

specs//

iMac 27" mid 2011

Core i5 3.1 GHZ

4 GB Ram

1 TB HDD

1GB Graphic Card

Hello guys, i have a iMac 27" 2011 model ,iv been having problems with this machine for a couple of weeks first it was flashing with all patterns and lines, then it had a white screen flashing, and then it just loaded the white screen with no chime, so i thought it was the hard drive so i brought a new one, i brought the machine new from apple, i brought it 2011/2012, so i guess the hard drive was on its way out.

so iv installed the new hard drive same specs, only difference is its 64mb cache. the other was 32mb cache.(both WD hard drives) i installed yosmite from memory stick fresh installation on new hard drive, been on it all day with no problems, and suddenly BAMM, it starts flashing and all different colours,and then the screen desktop was moved halved so the mouse would appear on the other side its so confusing, and now it half loads the loading bar, then it goes straight to white screen, and restarts, I'm so annoyed really annoyed, spent money on new hard drive and all day sorting it out,

Please can someone help me?

I think it may be graphic card ? or software problems ?

ps. Sorry for bad spelling and gramma, my dyslexia playing up and its late.

please help me folks. got a deadline for uni work in a week and I'm stressing to ****.