video freeze, sound is ok but only when lan cable is connected

I have an annoying issue I can't solve: when the device is connected to the Internet with a LAN cable I get continuous video freezes on all sat channels but also when i play video from a USB flash drive which is plugged into the device. Continuous means every 4/5 seconds the image freezes for 1 second and then goes on. The freeze is ONLY video, not audio.

This happens with all main images (OpenAtv, OpenPli, and OpenVix).

If I disconnect the LAN cable, the issue immediately disappears.

If someone else has a similar problem? How I can resolve it?

it turns out that it is something with my router asus rt-ax55, when I plug WAN directly into the box then it works fine. but when the router is between WAN and box then freezing appears.

anyway i do not understand why...