06 f350 6.0 taillight problem
 

The other night i was driving and i noticed my left rear blinker light was out. I took the taighlight off and found no issue with the bulb. Then it started blinking normal. I replaced the bulb anyway, the next day i was told my left brake light was out. I checked the light again and for chaffed or loose wires which i couldnt find. Then after i turmed my key on and applied the brake it worked. My left stop light works intermittent. Can anybody tell me if the socket is bad or is something worse happening