Maybe a scratched disc? Have you tried to clean it? Does it appears always after the fight or randomly?
Have you tried to clean the lens with a soft Q-Tip and water? It might help (at your own risk!). I rescued two Dreamcast consoles with this "trick".
If it appears randomly:
Sounds like a heated connection between the mainboard and PSU. It's a known problem. Open your Dreamcast and put some toothpicks in the white plastic...thing (bottom left between the power switch and capacitors). That'll do it. If not you better solder the connections.