....
it's obviously a console/hardware fault, one you may have to get you hands dirty on.
it's very possible some chip (or pcb part) is on it's way out?
dreamcasts go funny when the video chip is damaged. (glitch graphics)
found a basic disassembly and cleaning video: Download Links:
Links are hidden from guests. Please register to be able to view these links. if you can get a cartridge teeth contact cleaning kit, will help.
the socket will need cleaning also.
check all circuits for continuity, including contacts on all side.
all the best. (some guys at retrocollect know their stuff)
