No, it is acceptable. It depends on his climate. If he is not running an Air Conditioner in the background, or if the area where he is is hot in general then it is fine.
I'm sorry, but if his GPU was that high due to ambient temperatures, the OP would be BURNING UP where he sits. Also, if it was due to his ambient, then wouldn't the OP have also reported his CPU temps being ridiculously high? I would only suspect ambient temps as a culprit if the OP's PC shuts off completely/has high temps, which they haven't reported as of yet.
According to Gamers Nexus, even the stock RX 480 (which runs a little hotter than the 470 mind you) runs a temperature delta of only 8 C. This would mean that the OP's ambient would be somewhere around 52 C. This article sums it up decently, even with its grammatical mistakes: https://www.ekwb.com/blog/what-is-delta-t/
OP, what model of GPU do you have? Some models, such as the Asus Strix, operate at those temps
under load, so I'd say those temps are a bit concerning.
@OP: When you say it restarts your PC, that usually points to the PSU. How old is your PSU? What model from EVGA's line up? There have been some bad apples in their line up. The 470 is also known to be power hungry.
Thing is, the OP's PC
doesn't restart completely. I would point to the GPU being forced to shut off due to a thermal issue. That would explain why he would lose the monitor signal but could still hear everything running on his PC. As smorizio recommended, I would look into repasting your GPU, and also testing it with something like furmark to see if the shutoff still occurs. Though, it would help to know the model of your PSU as well, OP.