Most of you probably have or will have this problem because from what I've seen on the road our cars are notorious for this. does your tail lamp lights act a little crazy when you apply the brakes? Your tail lamps are on but when you hit the brakes one tail lamp goes out while the other lights up bright, then when you test the bulb, its fine, & the locking socket seems ok too. well, its not. its actually the screws that secure the sockets & green wires they become a little loose with time. & if you tighten them, they'll just loosen up again. too tight, & you'll strip the plastic housing they secure too. just put a little thread lock, carefully tighten & forget it.
-vic