Ha! Got a Campy front mech just like that! Was having the same kind of problem, thought I'd adjust the cable, tightened the bolt carefully but it just came off in my hand. I replaced mine with a Centaur too, no further problems. I reckon the arm was weakened, maybe from having been dropped at some time and was flexing/bending under use so that the tension didn't release when the cable was slackened by changing down. Has anyone else had one break in the same way? Mine was a Mirage, but looks the same as your older Chorus - probably the same mech design moved down the range? Could there be a design fault or duff batch of castings from Campagnolo?