I bought a '91 Giant Yukon frameset from a bike co-op in Ohio, brought it down here, and assembled it. Not having a 36 mm headset wrench, I had it professionally adjusted, and I watched him do it. 40 km later, it's so loose I'm afraid to ride it home, but have no choice(I was only 15 km out). I fiddled with it myself, this morning, and could only get the slop out if the steering was also way tight. So I set it tight, put loctite on the threads, tightened down the locknut, and then backed it off incrementally. I know this isn't a proper fix, but don't know what is. New fork? New headset? both?
It loosens with the application of the front brakes.