happened on my DK as well when I got it.
the solution is actually kind of easy. if you know how to remove ICs, or know that when you face such a problem you generally try to reseat the chips... well if you tried that and it didn't do anything, there's another thing to check.
take the socketed chips out one by one, on these there's only like a small handful of them, but just make note of where you're pulling them out of. and check the legs on them, if they look dark then there's some corrosion on them that can prevent them from performing like they should.
I told someone to try this on a game last night... take some very light grit sandpaper, like 220, and sand the legs with it until they're shiny again. make sure to wipe them off before reinserting them, paper towel shouldn't hurt. also make particular note that you don't bend the legs.
check that, try that, maybe you'll get lucky like I did. I think the Z80 chip was really dirty on mine.