Please help...what is wrong with this?

I just bought my 5D last week...used. It had been serviced by Canon in Sept, mirror fix done, back focus issue fixed, nothing else repaired, and she hadn't used it since it got back from service. It's worked fine until last night (that I can tell), then I notice this weird band across the top of most of my images.

I have formatted the card I used, tested a different card, and it's still doing it. Not as noticeable, but it's there.
Please help!