Thanks to scsu, replacing the fixture solved the problem. Purchased a new one (Amazon, $200, 3 days), and struggled with the PITA T-25 screw. Not sure I understand the wiring. 6 pins, 5 bulbs, no common ground or return. Not sure why the old fixture didn't work, everything I looked at said it was OK. But the new fixture works, so I guess I'm good.

Thanks