Chain skip is the classic symptom of a worn out freewheel. Generally speaking, the wear on freewheel teeth isn't visible. BITD, you knew it was worn out when the chain started to skip. This typically took about 7 or 8 thousand miles, maybe a little more, depending on lots of factors. If you were a cheapskate and worked in a shop like me, freewheel life could be extended by replacing the offending/skipping cogs only. Typically it was the smaller ones that got used a lot during base training: a 15t and 17t for example.
This is why buying used freewheels is a coin toss.
Early Shimano freewheels, IMHO and IME, were not long lasting compared to competitors at the time. (They got much better later) I'd say there's a good chance those ones are toast. However, you could play around with your chain length and wheel position in the dropouts and the B screw if you have one. If you can get a bit more wrap you might be able to correct the skipping for a time.
For sure check your hanger. ^^^ A bent hanger will cause the same symptoms.
Last edited by Salamandrine; 02-05-19 at 11:56 AM.