Hi TDM:
Hard to say without more info...is your Sub carburetted?....what kind of carb do you have in it? Does it run fine other than the random dying? Is it hard to start when cold? What is the dying like?.... is it instantaneous or is there a slow sputtering and bucking till it quits? Does it start right up after dying? How long has this been happening?
Regards,
Henry