DURING A TRAFFIC REPORTING FLT FOR A RADIO STATION, THE PLT NOTED THE ONSET OF A SEVERE VIBRATION WHICH HE FELT THROUGH THE ANTITORQUE PEDALS. HE STATED THAT WITHIN SECONDS, THE HELICOPTER LOST DIRECTIONAL CONTROL & BEGAN A RAPID, UNCONTROLLED SPIN TO THE RIGHT. REPORTEDLY, THE HELICOPTER TUMBLED OUT OF CONTROL FOR A TIME. HOWEVER, THE PLT WAS ABLE TO REGAIN PARTIAL CONTROL AS IT ROLLED THRU A RELATIVELY UPRIGHT ATTITUDE. BEFORE CRASHING, THE RATE OF DESCENT WAS SLOWED WITH FULL COLLECTIVE & A ROTATING IMPACT OCCURRED IN A HORIZONTAL ATTITUDE. AN INVESTIGATION REVEALED THE TAIL ROTOR DRIVE SHAFT SPLINE COUPLING, PN 28-13609, HAD BECOME WORN. THE TEETH OF THE COUPLING WERE WORN AWAY & THERE WAS IRON OXIDE & CARBONIZED LUBRICANT WITHIN THE COUPLING. THE SPLINED MALE END OF THE COUPLING WOULD ROTATE WITHIN THE SPLINED FEMALE HOUSING. IT HAD ACCUMULATED 149.1 HRS SINCE LUBRICATED. THE LUBRICATION CHART & CHECK SHEET (PAGES MM-3-5 & MM-3-6 OF THE MAINTENANCE MANUAL) SHOW THAT THE COUPLING SHOULD BE LUBED AT 100 HR INTERVALS