Chosen Solution
HI, I have this iMac that freezes after a couple of minutes of operation; the mouse completely freeze and thera are color pixels everywhere random on the screen. I have teared the system down and changed the thermal paste on the graphic card (nVidia GT130 512Mb), the CPU and the chipset as well. The system has been cleaned from dust and I’ve re-imaged the hard drive with a new version of El Captain OS-X. Can it be a faulty Video Card?
Did you run any diagnostics? How to use Apple Hardware Test on your Mac. If you can’t get it to come up, are you even able to get the system into Target Disk Mode? Share files between two computers with target disk mode so you can see the FireWire symbol on your screen. Let us know what you can or can’t do here. Good Luck! Additional reference: Startup key combinations for Mac
