Thanks for the thoughts everyone. It's not the amp! It operates 100% without a hitch into every other speaker I have. I am pretty sure the only thing that will screw it up is if I keep plugging it into this cab!
I did another test... took the amp off standby for a couple seconds and got the same thing. Speakers randomly oscillating with slight noise, my entire power section fickering. After that I plugged the head into my ENGL 4x12 cab and everything worked fine, sounded dreamy.
Could this be a symptom of a bad wiring job perhaps? Here is my rendition of a wiring diagram for this cab-
All speakers are 8-Ohms. The positive wire is connected to the "inner" part of the speaker jack, negative on the "outer." The jack itself looks a little old and grungy. (also note: my amp only has 8-ohm and 4-ohm taps, if that matters).