not knowing the model and being kinda unfamiliar with the mid century stuff, my guess would be loose bobbin/case.
From a purely mechanical troubleshooting point of view... I would look to see what might flex under load. if it does at low speed, then high speed could be worse.
are the "skipped stitches predictable? in other words can you know that at x speed over y distance you will get z stitches dropped? if it is constant in any way that can help point to the source.