Ok, I had a 4 x 2 input NAND chip lying around, so I made a buffer out of that. The vertical banding has gone, now just the dot crawl remains. This might be due to my LCD tv (though I've not experienced it on other consoles to the same extent).
can you post part numbers and a circuit layout/picture of this?
I'm curious how this would behave in an NTSC 7800.
I'll try, but the board layout of the PAL rev C 7800 is quite a bit different to the NTSC layout