Spontaneous reboots could be a power issue. Note that statements about minimum recommended PSU wattage are just rough guidelines, as there could be any number of power-drawing components in a given system, besides the GPU, that we don’t know about. Plus there are PSUs of different quality, and like all electronics, PSUs age. In particular electrolytic capacitors will degrade, more so in hot environments.
The first thing to check is if there are any obvious interruptions in the power supply, such as power cable plugs not plugged in all they way, or PCIe cards not properly seated and secured in their sockets. There could be also be the possibility of cold solder joints, but unless you already know what those are, it would be difficult to explain how to spot them. Plus I have not one encountered one in many years, they seem to be rare in modern electronics manufacturing.
As for PSU recommendations, I usually suggest using those rated 80 PLUS Platinum (they are very efficient and often of superior quality overall), and sizing them such that the total nominal wattage of all components in the system (CPU, GPU, system memory, mass storage, other I/O) sums to about 60% of the PSU’s rated wattage.