Others who have reported this problem have found it to be poor electrical contact in one or more of the turn signal bulb sockets. I would say to remove all 4 turns signal bulbs and inspect the sockets. Remove any corrosion from the contacts. Also pay attention to the ground connection for the side of the bulb. Bend the contacts in the base upwards a little to ensure good contact. On the dual filament rear bulbs, make sure they are installed in the correct orientation for the deep vs. shallow bayonet tracks.