The problem is almost surely corrosion or burning in the sockets. The original sockets were poorly designed, but the solution is really pretty simple. Take out the bulb and clean the contacts in the socket with fine emory cloth or extra-fine sand paper. Clean the bulb, also. Flush out any dust with canned air. Apply a very thin film of dielectric silicone grease and reassemble everything. You should have no more problems. This worked for me, and I'm in Mississippi, the land of humidity.