so many things to check,
when it stops, jump off and check to see if you are getting fire to the plugs.
sometimes the old coil can get hot and as soon as it does, it expands and a broken wire will separate, no spark. when it cools down it will start again, do not ask me how!!!!
give the old tractor electrical system a good tune up.
then check fuel flow when it stops,
I placed a inline fuel filter between the cutoff and the carb to be sure I had clean fuel flowing to the carb, tank can be dirty, fuel transfer transport tank/cans can be dirty.
it just never ends.
always use a funnel with a very fine filter when pouring fuel into the tank, be sure it has a vented fuel tank cap, when it stops open the fuel tank cap and listen for a whoosh sound (air rushing into tank) if so someone has placed a non vented tank cap on the old tractor, must have a vented gas cap.
again so many things, will just take time.
let us know what you find wrong.
we may need the information next month.
ask an old farmer buddy that work these old tractors hard back in the day, to come over and look at it, many times they can see things that you have looked over 20 times. its happen to me before.