The screen flickers ever so slightly, some times it's a really bad flicker and sometimes it's only slight. In fact, it's almost playable. I guess it's an internal video problem though, because it's not random. To describe what I mean, take Probotector (Contra Hard Corps to those outside Europe). The first load up screen won't display, but as soon as it gets to the main menu the screen will show. There will be a slight flicker. Then during the character select screen it will flicker more heavily, but then during gameplay it will only be slight (and in fact is almost playable if you put up with it). But during gameover screens, the screen goes down vertically in a loop (if you understand what I mean). Any game I play, there seems to be some sort of pattern to the screen issues.
I've just bought a dirt cheap replacement console (I had no idea mega drives were so cheap - hell I may as well buy 10