I don't think the card is overheating.. it just seems odd. I believe it's artifacting? Not sure.
Card only running at around 57 degrees. Seems odd that it's artifacting.
C2D E6400 @ 2.2GHz
XFX HD4850 512mb
450W generic PSU
rest is filler.
Yeah, it's definitely not a heat issue, a video card wont start artifacting like that at those temps.
I don't think it's a power issue either, 450W is the minimum for a 4850 so you should be good.
Do you have the newest drivers? Go here and double check: http://www.amd.com
Should be Catalyst 9.7
That's all I got. Not sure what could be causing your issues, honestly.