you have a problem for sure it is a loose and probably frayed wire that
shorts out or loses contact by vibration from driving. I would get out a
ohmmeter and start wiggling wires. I had a truck once that had a wire hit
against the frame and did stuff like that. It took me a whole day to track
it down under the truck. Get a wiring diagram and start chasing all wires
going to the tail lights, dash, and turn signals, find a common point where
the circuit is the same for all of them.