Hey!
I'm pretty sure that won't disqualify you since on average you'd have been working more than the 20 hours/week. You explain your circumstances in an essay and then of course you'd need your employer to state how much you've worked. Honestly though, I would leave out mentioning those weeks that were under the 20 hour minimum if they really were just single weeks here and there since the majority of your weeks were above that minimum.
I hope this helps