First thing I was going to say is what rev the buster was, but you covered it in the end of your post. The best advice I can give is to see if you can find someone that also has an A4000 you can send it too for testing. If its the card they are hard to fix.
If its the card they are hard to fix. My advice would be to get an indivision MK II CR for the flicker fixer/scan doubler and one of the new MNT cards for the video modes. Its a better card in my opinion and very nice. It even had an SD card slot on it to use for storage like a HD.
http://shop.mntmn.com/product/mnt-va2000-amiga-graphics-card-zorro-ii-iii
