Did you actually check the card for cold solder joints before freezing it, or did you just hope the cold would magically fix whatever was cracking? Because 3 weeks is about the same window you get from a half-assed reflow, which tells me the freezer is just shrinking the board enough to make the broken connection touch again. Next time it artifacts, try pressing on the back of the GPU die while it's running and see if the screen clears up, that'll tell you if it's a solder ball issue or something else. How hot was the card getting before the artifacts started, and did you ever check the VRAM temps?