I had this exact same thing happen to mine. I bet if you check you will find that the turn signal is indeed flashing fast on the outside. The reason the turn signal works properly sometimes it that the broken filiment just happens to bounce around and land in a position to complete the circuit temporarily.
Change the bulb and your problem should go away. If I were a betting man I place money on one of the front bulbs being bad.
Change the bulb and your problem should go away. If I were a betting man I place money on one of the front bulbs being bad.