... a Noblebone Bow, it's a Devilbone Bow. The Devilbone Bow looks exactly like a Noblebone Bow, and is the top of the range of the "normal" bows. It does 5d2 + 7 damage, and can be enchanted to have other properties in addition.
in the version of the game I have, my master @repair item fixes it with no problem when it breaks, which isn't very often. Seems to be a constant respawn from one of the chests. I don't exactly remember where, though.
shoots +12 and does 5d 4 +12 damage. It will say that it has the special bonus of 'Explosive Impact'. To the best of my knowledge, the Noblebone bow cannot be enchanted. I could be wrong, but my caster can't enchant it. The Devilbone bow, on the other hand, can be enchanted. I've done it.
If your bow had an enchant, maybe it was not the one you think it was?