Just an update;
I flew to my A&P to have them look at the charging system, as you'd expect, there were no charging system issues on the flight up.....everything was nominal. After several attempts to solve the problem, all they found was a defective "Alt Out" circuit, which was subsequently repaired. They indicated it may be the master circuit breaker (by the pilot's right leg on the side of the center console), but that checked out OK as well.
So I'm still left with this mysterious problem.