I recently made some modifications to an older bike, and in a nod to modernity (and steep hills and aging legs), I converted from a 7-speed freewheel to a Harris cyclery custom 13-34 9-speed cassette. All went reasonably well, except the Huret duopar (arguably the best-shifting touring derailleur of the mid 1980's) shifted very badly on the new cassette. Or maybe it shifted too well -- it changed gears very easily, but it wanted to keep shifting, and it was nearly impossible to find stable spots for any gear (for those of you coming in late, bear in mind that none of this equipment is indexed). I tried a shimano lx (robbed from a early 90's mountain bike that I don't ride much) and it worked just fine.
The duopar has worked just fine for twenty-odd years, although the last time I put on a new chain and freewheel I began to have hints of this problem.
Has anyone had a similar experience? Any ideas about how to make the duopar work better on this setup? Or am I stuck with Shimano?