There could be several causes, the most probable being a "poor" connection somewhere in the circuit. A poor connection could include a loose connector, corrosion build up at any connection, an overheated/"cooked" connector, a cut or broken conductor [wire] which occasionally [as a result of vibration] comes [temporarily] BACK into CONTACT, or some similar intermitant interruption of the electrical current flow through the circuit, and of course, a defective turn signal switch. Often, a loose or corroded GROUND connection is the cause of A SINGLE light operating intermitantly, BUT IF ALL the lights in the turn signal system are involved, then that suggests that the fault [whatever it is] is located somewhere between the power source [the battery], and the downstream [outlet] end connector of the turn signal switch. If the fault were in the wiring downstream of the turn signal switch, then I suspect only certain part[s] would be affected. j3h.