A couple of months ago I had to replace the headset on one of my bikes. During the process, I discovered that the fork has non-standard Raleigh threads (the bike is a late-70's Raleigh Grand Prix) , so it can't take new headsets.
Fortunately, the mechanics at my wonderful LBS rooted around in their back room and came up with a pile of used Raleigh headsets. With a little trial and error, we were able to mix and match parts and come up with a complete headset.
All was well for about a month, when I noticed the headset was loose again. I tightened it up, but a week later it was knocking once more. Since then I have to tighten it about once a week. Last time I regreased the threads and cranked the locknut down as tight as I could, but it still came loose again.
What could be causing this, and how can I fix it?
Despite the headset's questionable pedigree, it works great when it's tight, and everything seems to fit correctly. If the headset is somehow defective, my LBS can probably come up with another used Raleigh headset, but a new headset isn't an option unless I replace the fork.