I'm still thinking your problem is the 3 wire going out to the bulb. The black and red are sharing a common. At some point that black needs to be splitting off to your power source and the red should be going to the bulb. So somewhere that common (white) is getting split to go with both wires. There should be a J box wherever that split is. (or it splits at the bulb junction). My money is on there being an issue there at that junction (corrosion). Usually buried wire in conduit is fairly stable (unless someone has been digging)