On Sat, 16 Jul 2016 19:45:41 -0700, Jeff Liebermann wrote:

The logic was easy. You had already done most of the obvious
troubleshooting (increase load, add capacitors, check currents). There
was only one active device capable of causing a problem.

False assumptions have always plagued me. In this instance I falsely
assumpted that because the reg was functioning fine under a heavy passive
load, there was nothing whatsoever wrong with it. I didn't admit to the
possibility of partial damage which could manifest under other load
conditions and that made me completely blind to fixing the problem.
Lesson learned - I hope!