I haven't been following the thread; but just jumping in at the end here and eye-balling it... I think...
I would suspect either bad VRAM, a fault in the data bus connecting to the VRAM, a bad buffer on the bus (if it even has one - I haven't looked at a schematic) or a bad video output chip.
Basically, what that vertical stripe pattern is indicating is that one of the bits in a byte (actually, it looks like a bit in two bytes, as it seems to be repeating every 16 pixels on the screen shot of black-and-white mode, and every 4 pixels in 16 color mode) of the video data is getting messed up. It could be that a whole column of the video memory chips is fried; or it could be something on the way to or from that video memory.