Steve called today from Outlaw to start troubleshooting by phone.
Typical suspects discussed included electrical damage due to lightning, brown outs, surges, etc. Then he asked about what else I had on the circuit (it's dedicated). From there we moved onto lighting. First, whether the room contained any halogen or flurescent lighting (nope). Second, whether we had a dimmer switch (yes, mechanical infinitely adjustable).
More probing revealed that the dropoff seemed to be limited to center channel dialogue which triggered his initial diagnosis of an intermittant center channel interconnect. The Rx is to swap out the suspect cable and see if that solves the problem.
Funny how intuitive a diagnosis is when you're posting after-the-fact.
However, since I have not verified this fix as of yet stay tuned for more details...