I think I've figured it out
I have the same problem.
You know how when you start games with the NTSC cable, the load screen starts out distorted, and then corrects itself later?
Well, that's not behavior that's standard on the Dreamcast, but most games set the resolution themselves anyway on start.
However, Jojo doesn't do that - it stays at the distorted resolution. And it's incompatible with the VGA cable, to boot, so you can't fix it that way.
The only thing you could, I imagine, would be to make a self-boot version of Jojo that allows you play it in VGA mode. I haven't tried it myself because I'm too lazy to bother.
An easier way would just be to wait until the next version of Chankast and hope it gets fixed. It's a software bug, and short of the above, there's nothing you can do.