Originally Posted by zandoval

Hate those Kreaks... Replace all and see what happens - Local guy after many months of searching found a damn near microscopic hair line crack in his head tube...

Knew another guy who fixed a kreak in his stem by adjusting his seat... Who knows... Good luck...

Originally Posted by Ferrous Bueller

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.