[Lil bastard]

If you remove the wheel and look at the caliper, you'll see a wire clipped into the retaining clip. Follow this wire back toward the car and see where it connects. Pull and reseat the connector, check for corrosion. If that doesn't work, take a DMM (multimeter) and pull the connector for a sensor and check it for continuity. If you don't have continuity, then you'll need to replace that sensor wire. Because of the randomness you speak of, I think 1 wire may be broken inside it's insulation, probably where it gets pinched by the retainer, sometimes it makes contact and you have no light, other times it breaks the circuit and the light comes on.