Low fluid can be. Especially up hills or on turns, when the fluid sloshes in antoher direction.

That magnet you describe is not the original, but should work fine. The potential reason you need more than 7-7.5 quarts is because you had it apart for over an hour. Over time the torque converter/other internals will drain down. It's not bad, just means you need more fluid to refill. After a week of sitting, I needed 10 quarts to refill.