Success!

Turns out it was a bad ground after all, of course. But not the ground from the lamp assembly to the body - the grounding (vertical) strip in the bulb holder wasn't making contact to the lower bulb (horizontal) strip. I knew it was going to end up being something remarkably simple.

Anyway, it turns out that a new bulb holder would've fixed the problem - but at much greater expense, and without me learning what the problem really was.

Thanks everyone for your helpful input.

__________________

I demand FULL CREDIT!!

Hahahahaaa.

Glad I was able to help.