iMac 27" (Late 2009) frozen with yellow lines / dots

Yesterday while loading my browser, my iMac froze with yellow dots. After a few minutes of trying to get something to work, I finally had to boot down by using the power button on the back. After a minute or two, I rebooted. When the computer came back on it immediately had a yellowish tint with thick vertical yellow lines (see picture). The Apple logo was in the center just as if it was rebooting with the progress bar moving. But once it got to about 3/4ths the way, it stopped and the screen switched to the small yellow dots again (see picture). I can not get the computer past this point. I've tried waiting; command+option+p+r (holding for three times); and also unplugging it for several hours. After the command+option+p+r procedure, now my progress bar only hits 1/2.

Help, I can't find any other options. Is my video card bad and the computer won't let it reboot due to that? Is it something else?

iMac 27" (late 2009)

MacOS Sierra (just uploaded a day or two after release)

2.66 GHz Intel i5

4 GB memory

ATI Radeon HD 4850 512 MB