Retrodroid, an odroid xu4
Thats tough, I don't notice a lot of XU4 users here, and I don't know anything about them. XU4 forum is a better idea.
Perhaps updating the kernel, to the latest.
Is there any sort of config.txt for the kernel options, like as on the raspberry pi? Usually for overclock, or hdmi configuration, also kernel overlays modules.
there is a uboot.ini for XU4, and I didn't look far into it, there may be some configuration here that simply works
How long of an hdmi cable just out of absurdity? I think a very long 50ft hdmi could have problems like that. TVs okay probably, but maybe there is 4k upscaling(?) interference, switch it to a "Game" or "Graphic" mode if you already haven't. Maybe disable 4k options if possible.