If I remember correctly you had very noticeable artifacts at stock OC in Heaven, and you tried Hitman since I said it is more similar to your actual gaming performance, right? And you said you had no issues in Hitman, and I think you said your temps were fine? What changed since then? Are you still on stock OC, did you switch drivers, or is this just occurring after a longer period of running Hitman?
If that is the case, and there are no better suggestions, I am sorry to say but I think that card need to be RMA'ed. You definitely should not have that bad artifacts in Heaven at stock, and if Hitman crashes you do have a problem.