Edivon, this is exactly the problem I've been reporting, with the exception I use Photoshop Elements instead of Aperture. Same as your case, slideshows (I'm not talking about screen saver here, that's a separate idiotic issue...) work perfectly fine in my ATV 2. But in ATV 4, when using Ken Burns or Classic if you set 5 seconds or more per slide, it just stops after a certain number of slides, and if you set it to 3 or fewer seconds per slide, it starts fine (only from the beginning - another issue), runs a certain number of slides sequentially, then it plays a few of the previous slides, then it resumes sequentially playing slides from the "go random point" (not sure but I believe it even skips one or more a few slides, I'll check that out), then at some other point more random older slides, and so on and so on. Really bad, annoying. Again, everything works fine, as it should, in my ATV 2.
I've tried everything I've seen posted here and everywhere, including those instructions kindly posted by CarlAnders, without any luck, all problems remain the same. If ATV 4 and tvos are what Apple sees as the "the future of TV" we may find ourselves watching Game of Thrones and suddenly random scenes from previous episodes will show up. Not cool. So far, from a user experience angle (and as a paying customer too...) I fail to find a single element I can say the ATV 4 is an improvement over ATV 3 or even ATV 4. On the contrary. It looks more like a regression. Siri, it's faster... Give me a break.