Check to make sure your problem is indeed (as I suspect) the chain skating over the freewheel cogs. I have this problem on one and only one of my bikes, Capo #1, which is equipped with a ca. 1980 13-15-17-19-21-23 Regina America freewheel, a ca. 2000 new SRAM PC-58 chain, and a ca. 1980 Campagnolo 980 rear derailleur. I do not have a good general solution beyond being careful not to apply too much torque until I am sure I have a positive engagement. The problem is worst on my most-used 19 and 21 tooth cogs. My next freewheel for this bike will probably not be another Regina.
__________________
"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069