First photo looks familiar (sometimes this line flashing) - that I noticed many years ago.
Last two... Maybe, I saw. I'll try tomorrow to load and watch.

Yep. Fast Food has same effect.

R12 - shorted. Nothing changed.

Here photogallery: https://www.dropbox.com/sh/oduseq994r1t9s2/atC8Cvyc5-

I don't remove PCB with memory, because I don't have currently soldering-iron to put it back.

Some photos from other side here: https://www.dropbox.com/sh/kckg3wa3inocrub/AgKLQn--Qt

I found same motherboard but don't see any crazy screen bugs.
It is good new for me But bad news about your problem hard to find what is the problem if I'm not see it .-(

I will try to make minimum version from the know "bug-generator" programs, for example only keep the LPT and screen build parts. If the minimum programs also make a crazy screen for you, then possible change something or other something, etc for detect which Nick settings generate bugs. If we know the specified settings then can thinking about what do this in hw level.

But it is will be later, I'm currently busy about the SD Cartridge software