Case Instability No More
Back in the saddle again with my desktop. It seems that the power supply was either faulty or not putting out enough power (perhaps even a combination of the two). With a new power supply the box has been stable so far.
This could have been such a simple issue to resolve were it not for the odd combination of symptoms.
In any case, I am glad to be able to run my desktop as trusty old workhorse again.