Repair or rebuild a stone wall
If the stones are sound and only the joints are failing, repairing the joints is usually the better call, because it keeps the wall you have and costs less than taking it apart. It's the same idea as brick pointing, done in stone. When stones have come loose across a whole section or the face is bulging, patching just buys time, so I'd rebuild that section. And if a crack is only cosmetic, I'll tell you it can wait.

