I see a lot of hypothesis but i'll make short if some didn't read previous post.I'm not the designer of the cartrige but if you encounter this room changing problem, check how fine your console has been re-assembled, especially mass and ground connect, it maybe not the ultimate solution but i'm pretty sure it is the most frequent issue, the other possible issue is vram reading problem as vram is used for collision détection but you should have some visual corruption in this case.
About testing, we tested a lot this game, especially about vram timing problems, playing with coleco joystick too, the game runs in various events through Europe, every problem detected has been fixed, some were really inexpected but we worked to fix them...
Sorry for the long post, to make it short : please check if your coleco has been correctly re-assembled after modification and take attention to grounds.