You have already received some proper information thus far. If I had to choose what to go after first and foremost, I would go with the electrical issue of either the coil or points and condenser.
All depends on how she “shuts off” does the shutdown occur as if someone turned your ignition key to the off position? Meaning it shuts down quickly as if it were a light switch being shut off… If she slowly dies however (as if running out of fuel) then it may be just that… a fueling problem.
We need to know which it is before any of us will be able to solve the problem. A quick shutdown will be electrical… a stumbling slow decline to shutdown will be a fuel flow related problem.
Let us know which it is… I am currently leaning toward an electrical issue (no pun intended).