It's certainly a missed frame, either that HDMI converter or the monitor is skipping it. The problem here is that the C64 does not output the exact expected framerate, the errors add up, and every couple seconds you see a "jump" because a frame is skipped. You'll get the exact same thing with an emulator that isnt framesynced, for example.
As said, its certainly related to the NTSC to HDMI conversion - you just dont see the problem in all cases, as it also depends on how fast things are moving, and what other stuff is displayed at the same time. Try something that shows different portions of the screen scrolling at different speeds, you'll see it clearly then (my testcases for this are the "giana sisters" or "starpaws" wait screens)
Searching back I first noticed the blurriness of scrolling originally on modern LCD/LED monitors using
emulators on the PC.
I'd pay for a product to get around this limit on my retro machines. Quality is worth the whole experience.
I'm going to try using a direct RF video in on the LCD monitor upstairs. I'm curious to see what the result is.
(I've been using svideo out of the C64RMK2 to an hdmi cable convertor). I have an old 8 pin din that carries
the audio channel but not chroma. It's b0k3d. lol It does get slightly better performance scrolling but it is
still, I say, half the blurriness of the svideo. We'll see.
It would be nice if it was the convertor cable because that'll open the doors to a potentially replacable parts
either manifest or commercial.
The replacement part could be called 'Nausea Buster'. XD