rpm is jumping up then goes all the way down and eventually dies.

rpm is jumping up then goes all the way down and eventually dies. Sorry for jumping in guys im new to this. I have a 1946 chevy with a 235 inline 6 not sure the year of the 235 but the car actually runs smooth. The problem im having is my rpm is jumping up then goes all the way down and eventually dies. I can hear some kind of vacuum leak also but cant figure from where. Could it be my vacuum advance? Thank u