I think it's a pretty different issue - mine doesn't have any garbled graphics, but it's missing stuff, like background images or title screen intros. As I mentioned, I tried Samurai Spirits in another console and it worked perfectly, but I have tried a couple of other games and experience similar issues shown in these clips (ignore the poor video quality, which is due to either a bad AV lead or PSU):
Metal Slug X:
https://www.youtube.com/watch?v=Ecs3j4upjz8&feature=youtu.be
Neo Turf Masters:
https://www.youtube.com/watch?v=R-xleLHNugI&feature=youtu.be
Samurai Spirits:
https://www.youtube.com/watch?v=D9aAlCbz_tk&feature=youtu.be
Note, these are all using a NeoSD, but the issues I got from the original Samurai Spirits cart and rom are identical.
Aside from checking the connections between NEO-B1 and the palette RAM, I've also checked continuity between the cart slot and the LSPC2, between LSPC2 and the NEO-B1 and between the 68K and NEO-B1. Obviously done a general visual check for broken traces and checked the video circuitry beyond the palette RAM, although I very much doubt the issues with stuff simply missing is unlikely to be caused by anything there. None of the chips are running particularly hot.
Any idea what to check next? As I said, the diagnostic BIOS says everything's A-OK. It seems to be distinctive elements that are missing from each game, so if anyone knows which cart pins they're related to, I guess that might narrow it down a bit.