For certain a 10 spd chain could not be helping. I'm surprised that works at all. +1 new on a new PC 850.

FWIW, the generally held view in the freewheel era was that you should never put a used chain onto a new freewheel. Folk wisdom said: if you put a used chain on a new freewheel, it would wear it out prematurely. Therefore, when you got a new freewheel, a brand new chain went on.

+1 for checking the freewheel bearings. That's more wobble than I'd expect from a Duracce FW, even the early ones.