1 - if under the load only, then overheat of a video ram/gpu
2 - anytime load/no load, then its already damaged memory/gpu, especially if you start seeing artifacts during boot process, before driver is even loaded.
i would not worry if gpu runs anywhere under 80'C and i would worry if it runs over 90'C in between is sort of "pay attention" area to my opinion
Me personally on my nvidia rigs:
<60* is ideal
60*-75* is acceptable
>75* is unacceptable
If they start to go above 75*, I start adding more active cooling to keep it in check.
thanks for this, you informed me well