Sapphire recommends a minimum PSU of 500 Watts for your GPU card which means your PSU is about 100 watts weaker than it should be.
I suggest you just upgrade your PSU to a metal rated PSU to at least 850 Watts. This will also give you some leeway in the future if you decide to upgrade your GPU or CPU to a more powerful version.
From this PSU tech site: https://www.cgdirector.com/psu-too-weak-for-pc/
What Happens If Your PSU Is Too Weak For Your GPU?
If your PSU is too weak for your GPU, a few different things might happen.
First and foremost, you may just find yourself unable to fully power on your PC with the GPU installed, or find that it only outputs a black screen when you use it.
If you’re able to boot into Windows without crashing, chances are your GPU is running under the lower end of its power limit…but once it’s properly stressed and tries to reach for the wattage that isn’t there, you’re most likely to see a full system crash.
I once experienced crashes like this when my power supply was plugged into a UPS (Uninterruptible Power Supply) that didn’t have enough wattage to match it.
Besides undervolting your graphics card to meet your system’s wattage requirements, the best thing you can do in this situation is to either get rid of some of the other power-hogs in your system (like a lot of storage drives) or simply find and upgrade to a stronger, more efficient power supply.
Found this Tech site that explain in details about all the possible issues you can have with an underpowered PSU with a overpowered GPU card: https://robots.net/tech/what-happens-when-psu-is-not-strong-enough-for-gpu/
I bookmarked this site due to the excellent explanation of all the possible issues your PC and GPU will have with a under powered PSU installed.
