Well everything I could find about the x650 was that it was indeed a single rail unit. I find it surprising that after 6-years of about a half load that it would be going bad, or that my new configuration is exceeding it's capacity. But, it is not outside the realm of reason so I'm going to try a new PSU.
I went with another Seasonic, the PRIME 750W Platinum: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I like the 12 year warranty but I'll probably just forget about it. The extra 100W might help settle any capacity issues, and the new unit will rule-out if mine's failing or not. Crossing my fingers this does the trick.
Thanks for the help everyone.