When you clean the contacts in the sockets also check the condition of the fiber support piece. Spring pressure at the edges and the bulb pushing against the "buttons" in the center tend to deform the fiber. Any moisture that might have been able to wet the fiber makes the problem worse -- sometimes to the point the fiber bends so much a reliable contact is hard to maintain. If worse happens, Rhode Island Wiring and others have new fiber supports, springs, and even contact buttons that will fit most of the sockets.
For any socket size where modern is not available a replacement for a rusted spring can be fabricated out of music wire or maybe something close found at a hardware store. A piece of thin phenolic sheet from McMaster-Carr and the old support along with some time with a saw, drill, and rasp will make it as good as new.
TXgoat, The light fixtures were polished, painted and new lenses and gaskets were installed. I dont know how I missed the corrosion in the socket. Maybe I didnt think it would be a problem... and yes the battery is always disconnected on cars that I am working on/stored inside/not currently driving.
Howard, the springs do seem to still have good enough pressure and the fiber board back there still looks good!