Check the length of your chain -- 24 half-links, from center of first pin to center of twenty-fifth, should measure no more than 12-1/16", which would indicate 1/2 percent of elongation due to internal pivot wear (so-called "chain stretch"). If the chain is too long, replace it.

Since you (understandably) use low gear all the time, and since that 22-tooth cog looks much more worn than the others, it may be time for a new low-gear cog. Consider replacing the 22 (assuming you have a 14-16-18-20-22 freewheel, which was very common back then and which I eyeball guess yours to be) with a 23 or a 24.