Whatever's causing that, it will probably be solved by the basic maintenance that a 49 year old bike needs. Possible causes:

--stiff link in the chain (if you find it, flex it back and forth between your hands until it softens up; then oil the chain and wash your hands)

--bent tooth on the chain ring (if you find it, bend it straight with an adjustable wrench)

--bad bearing in one pedal or the other (put the bike on a stand and pedal by hand with one side, then the other; notice any difference? Well, your pedals should be overhauled anyway...)

--bad bearing in the crank (this is probably the problem. You will need to overhaul the bottom bracket. This is a bit of a pain in the neck, but be of good cheer: regardless whether this is the problem or not, you really should overhaul the bottom bracket. You will probably find the spindle and/or cups worn, one or more balls are chipped, etc. Hopefully just the balls).