For the same reason my old Fiat lights and gauges were whacked out until I drove it again for a bit.
Crappy connections, and I totally agree that your issue's sound like connections.
Especially grounds.
I have put quite a few dedicated grounds to my light housing sockets just because of issue's like you are having.
Some I grounded right to the bulb sockets because the ground path from the socket to the light housing was junk.
Corrosion gets built up between the two metals (pot metal and the steel or tin sockets.
