Originally Posted by
Oldskool
Open it up and check the modding. Make sure that there is no damage to anything where things were soldered. Or even un-mod it for now to set it back to stock and see what happens.
As far as a know, when the Sega CD is region switched, it has to have a separate BIOS for each region installed. There might be some sort of an issue with the way everything is working together, especially since your problem is with the BIOS. Unless there is some custom flashed, menu driven regional BIOS that's installed. Not exactly sure how all that works.