Actually quite easy to have an intermittant turn signal, as lamp socket connections can get corroded...moisture + two dissimilar metals = battery (material and electrical transfer). Check the lamp socket contacts, the solder connection (bulb "nub" and base) along with wiring on back (including the tail light/license light assemblies. Common for water and abrasion to cut through the insulation and turn copper wire green. Clean with knife or file, and put on dielectric grease to resist water intrusion and corrosion of all electrical connections. Auto parts store or HD dealer has it in stock.