I have a light fixture in my garage, and when I put a normal
incandescent light bulb in it, I cannot reliable turn the light on.
That is, when I flick the switch to "on", I can see the bulb turn on
for a fraction of a second and then turn off, and stay off. If I
repeat this process a few times, the light will eventually stay on for
a few seconds, but then turn off again.

The fixture does not have a light sensor in it, and it was working
reliably for years until recently. Can anyone give me a clue as to
what's going on?