You need to look further downstream for something that is locking up.
Obviously, it's happening intermittently, since you're able to turn the
cable with a drill and everything works. I think there's a small reduction
gearbox between the two cables. Since it's the lower cable that's breaking,
the problem is probably that the reduction gearbox is worn, and from time to
time the gears inside lock up. Since the transmission is still supplying
torque, but the cable can't turn, something has to break. No sense making a
stronger cable - it will just cause something else to break.
Egon