I just looked at nvidia-smi myself. I thought it did diagnostics, unless I'm missing something, I just see it as a sort of setup program that can monitor gpu temp. Since you had your problem, I realized it would be nice to find a gpu diagnostic app that works with linux, everyone I find seems to be windows only. Going to keep my eyes out for one, maybe look a little deeper on nv forums over the weekend, or maybe see if there's something on my Hiren's CD that will do the job.
I'll let you know if I find something.