Hi Jomsp,

If you say that this happens always on the 8th, 16. or 24. position, I would say that this is independed from the content itself.
So now it is very important to check if it is the Server(PC) or the Client(TV).
So please test it via USB and also, install an alternate UPnP Server on the PC:
In this case i would recommend fuppes. It is an easy to use server, after installation, just share the folder with the images and check if this behavior is still present or not.