The derailleur is "hunting" due to not being properly aligned to the rear cog (not sprocket) that it is under. To have that occur only in certain cogs strongly implies an alignment issue, either the hanger or the derailleur itself is bent. You must first make sure that the hanger is aligned, and given the apparently severe trauma your derailleur went through (helps to know what happened) I would say you need to have it professionally aligned, as the hanger may well be twisted rather than just bent inward. If it is OK and you still have the problem again I would suggest switching out the derailleur from the identical bike.