I HAVE A 1981 CHRYSLER IMPERIAL.WHEN IT STARTS UP AND PUT IN GEAR IT DRIVES OFF NORMALLY. AFTER STOPPING AND PROCEED TO GO THE TRANSMISSON ACT LIKE IT IS SLIPPING AND IT VIBRATES AND MAKE A GROWLING SOUND. AFTER DRIVING A FEW MINUTES ALL SEEM TO WORK WELL. CAN YOU HELP ME? THANKS IN ADVANCE