I had the same issue on my 82 300d a couple of years ago. At first the flashers would occasionally fail, and I could "reset" them by turning the hazzards on and off. After a while this did not work and I could sometimes hear buzzing from the flasher relay when the flashers failed. The solution was to replace the flasher relay (located just in front of the shifter, under the console wood. I have no clue why cycling the hazzards worked temporarily. The flasher relay was about $50 from the dealer, as I recall.