Torque converter

Hey everybody!
I have been a member for a long time and maybe its because I'm getting old but I tried the search function to look for starting problems and couldn't find what I'm looking for,

My 74 D200 w/360 and stock ignition cranks but will not start sometimes but when I let of on key it will attempt to start. sometimes it will start, but most times it will leave me sitting!
I cant remember the obvious cause...can u guys help an old (71) Mopar guy out?