I've experienced several aluminum frames stretching and ovalizing at junction points. I'd bet this is the case here.
Addressing stretched tops of seattubes, I've had success finding a tighter fitting longer seatpost. Maybe you can find a quill which fits a bit better. A precision caliper would be useful here, or lugging the bike down to a shop which stocks these older stems.
Keeping the contact areas completely grit free is also key. Make sure you've got the inside of that steerer absolutely spotless when regreasing.