Ahhh... NTSC (Never the same colour) and that crappy 525 line resolution that plagued USA for decades, while at least in Europe we had PAL (or SECAM for the frenchies) with 625 line goodness.
... at 50Hz which was maddeningly flickery unless you had a costly frame-doubling 100Hz TV set (in CRT days).
The ideal format would probably have been a 525 line, 60Hz PAL. Who knows, maybe that's in use somewhere!
Except it wasn't - indeed it was interlaced so only 25fps, and it was scientifically proven back then that nobody could possibly notice 25fps vs anything else! (I was taught this in college!)
I do remember the awful american NTSC shows that had been converted to PAL back in the '80s - they had this terrible quality to them, not just the poor upscaled resolution, but like they were actually running at 10 fps. However I think that may have been down to the US studios storing everything on VT which was shit even at professional levels while in the UK everything was still recorded on film...?
Ah, good old British anti-American bias! No, US studios weren't storing everything on VT while the British used film. No professionals produced prerecorded TV on VT until the late 1980s, when VT actually became good enough to retain decent image quality in editing (and even then it was rare in big budget TV). Everything was on film, because only it had good quality. That's why we can watch the original Star Trek in HD now: you just re-telecine the film at higher resolution.
VT was originally used only to record live TV, which before that was either not recorded at all, or was recorded by filming a TV.
As for why converted TV sucks: 25<->29.97Hz frame rate conversation is essentially impossible to do without artifacts. The 10Hz effect you describe comes from the 4.97 dropped frames per second. It doesn't divide evenly so you get judder. (For content originally shot at 24fps, they instead run the film at 25fps, which then has no judder but does run fast and with a slight change in audio pitch, which I can readily detect in content I'm familiar with.)