Curious. You may want to ask about this over at Sega-16, as there are a great many people familiar with the hardware of Sega systems.
My weirdass problem is with my Atari XEGS. When I press the return key while in the BASIC environment, the whole system locks up. The return key works normally when in the keyboard test screen, and doesn't lock anything up when pressed in a game or in other test screens. I suspect the BASIC ROM is fucked up, but I have no means of confirming this.