Joined: Apr 2007
Posts: 12,563
Likes: 2,739
From: Thunder Bay, Ontario, Canada - burrrrr!
Bikes: 1958 Rabeneick 120D, 1968 Legnano Gran Premio, 196? Torpado Professional, 2000 Marinoni Piuma
Two possibilities...
The headset is too tight or the bearing races are indexed, or dented, forcing the balls to fall back into the dents.
Were it me, I would disassemble the headset and look for the indexed or dented bearing race. If all is good, assemble and adjust to eliminate slack and prevent binding. If the headset is indexed, replace it. There are tricks to avoid replacement but don't recommend them.
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"98% of the bikes I buy are projects".