when you have a sporatic condition you have excessive resistance somewhere. if its the same side as the taillight that was replaced then look into the socket where the turn signal bulb is. the bulbs are usually sealed to the taillamp to keep them dry. if you got water inside the socket you probably now have rust on the contacts inside the socket and that would cause the sporatic operation.also, check the bulb. the bulb has two filaments, one may be on the 'way out' and the other good. changing bulbs side to side and see if the problem goes to the other side. if so, just changing the bulb will usually fix that.lastly, I don't know if the ground wire was disturbed when the lens was changed. the ground is through a black wire to the sheet metal inside the trunk. if that got rusted or corroded you'll have the sporatic operation.my guess is the bulb is bad followed by rust in the socket or on the groundhope that helps