Titzio lamp glowing faintly ?

I have an old model Titzio 50, that after almost 40 years suddenly only glows faintly. I changed the bulb with original Osram H1 (12V 55W P14.5s) but got the same result. I chequed the old and new bulb with voltmeter and both are fine. The lamps shine bright when fed 12V directly. I measure no resistance between the rivets on the left and right leg of the lamp, but am not sure where I should find discontinuity for the two. There is no humming or visible flickering. The lamp was changed approximately 30 years ago from 220 to 110V (EU to US). I would like the lamp to shine bright again instead of the low glow, and would prefer to stay with my old lamp because the new versions are made from plastic instead of metal (really??). Any idea where I should start looking to identify the problem?