Arans are supposed to have low damage that builds up to higher damage as their combo counter increases and as they combo different attacks together. You can do more damage on all of your hits if you upgrade your equips, but you will still have a few low hits that build up toward bigger hits. That's how Arans work.
If the instability really bothers you, you could pick a more stable class, like a bandit, although no class is really stable in 2nd job.