That doesn't look like a Vertical Sync problem to me, as I remember seeing them, although it wouldn't hurt to look for a Vertical (세로?) dial and try it.
I did a few minutes' research and found there's still a market for salvaged/recovered/renovated CRTs in retro video gaming community, and that in some instances, CRTs are very much in demand because some of the old games designed for CRTs don't adapt well to LCD technology.
https://www.theverge.com/2018/2/6/16973914/tvs-crt-restoration-led-gaming-vintage
I actually came across an example recently, when I went shopping for an Arcade game, House of the Dead, that I was considering buying because I had so much fun with that game in the early 2000's. Unfortunately the only HoD game I could find for what I was willing to spend, had a CRT that seemed too small to let me aim accurately (I'm not your typical spray-and-pray shooter). Anyway, from what I read, the targeting technology depended on using a
CRT screen, and finding and buying a large CRT would have driven my cost through the roof. To my surprise, I found almost 47,000 items when I searched CRT on eBay, and some of them are pretty cheap,
for the CRT; but shipping a CRT is darned expensive.
Anyway, GBboy, keep checking in here and maybe some expert in the retro gaming community will weigh in on your problem.
EDIT: Oops, GBboy, you posted while I was still composing. I'm going to leave this post here anyway.
Out of curiosity, are you posting from Korea? I was wondering if it was easier to find CRTs in Asia? I know the CRTs were on the market longer there.