basroil :
It's actually a bit more complicated than just that, since most GPUs nowadays use dynamic scaling of voltages. If you undervolt it but keep clocks high (as close to reference as possible) and it stops crashing, it certainly can be a wonky card.
When I undervolted it but kept clocks at stock values, I got a reboot after 5-10 seconds of running the test. Other configurations result in either instant reboot or no reboots during the whole test. Also using Force constant coltage setting in afterburner didn't make any difference.
basroil :
Try seeing if 3dmarks tests show artifacts at normal clocks, if they do, it's a sign that the card is starting to go the wrong way.
Also got a reboot during the demo part of 3DMark 11, then tried Benchamrks only and got a reboot again.
I tried running a test for 20 minutes with stock GPU Vcore (1.012V), but underclocked core (500Mhz) and memory clocks (1500MHz) . I noticed there are some interesting things happening with the plots. Mostly the CPU usage was 100%, but GPU Vcore was 0.95V, core clock 405 Mhz and memory clock 325 Mhz, but every 5 minute for 10-20 seconds the CPU usage would drop to 30%, GPU Vcore rise to 1.012V, and core and memory clocks would reach the set values (500MHz and 1500MHz).
Then I inspected the plots and noticed many unordinary things happening in those intervals. I will provide a link to all these plots at the end of the post, but here is a summary: It seems CPU Vcore rised by 0.05V when CPU usage dropped to %30. ( Note: I later tried with an overclocked CPU, with all throttling and monitor features turned off to make sure it wasn't intentionally done by mo-bo, and seems it wasn't.) Also the AVCC (+3.3V) seems to fluctuare between 3.33 and 3.31, but when the CPU usage drops to 30%, AVCC stays perfectly stable.
In the next test I did with the overclocked CPU (from 3.3Ghz to 4.5Ghz, with throttling and monitors turned off) the plots were very much alike. This behaviour might be intended by the test, but I don't think they would want to run only CPU or GPU at its max at a time when they are testing PSU stability. Anyway, here are all plots:
4 unordinary plots : http://postimg.org/gallery/2vf5i6ufk/a9932864/
A screenshot of the behaviour: http://postimg.org/image/xvifwrjcj/
All plots for CPU at stock: http://postimg.org/gallery/2mjdqj31m/b3a80cbb/
All plots for CPU overclocked: http://postimg.org/gallery/sfxikng4/fc06245d/