Just to do my due diligence and mark this thread as "solved":
Yes, the Asus Sabertooth 990FX R2 can most certainly handle an FX-9590 chip with no problems whatsoever.
However, upgrading from an FX-8350 to a 9590 is not recommended IMO, since an overclocked 8350 can match or surpass the 9590.
Ultimately, my solution was to upgrade to Ryzen. And ever since then, I haven't had any unexpected troubles with FPS.