I got through it all right until Level 4. At that point, all of a sudden, no matter how hard I tried to memorize enemy patterns and so forth, there was just no progressing.

I figure one day, whether I've beaten it legitimately or not by then, I'll buy a GameShark Lite and cruise through it and unlock everything. I'll go for infinite lives instead of invincibility, though; I wouldn't want to cheapen it.