Raspberry Pi composite video not stable, jittery image (PAL, sdtv_mode 18)

[Xitee]

Hi,
I am using the composite output of the Raspberry Pi 4b for my old CRT TV.
It's a 50hz TV (PAL).
First I've set the sdtv_mode to 2 but there was massive flickering / jittery text.
After switching to mode 18 it was much better, but the text is still not perfectly stable and there's always a bit movement in the image.

I've confirmed that it is not the TV itself (as I first thought) by connecting an old SNES to the TV. The pixels 100% stay in place and do not move.

I found out that when using sdtv_mode 16 for NTSC, the image is stable just like with the SNES. But because my TV only supports PAL, the image itself is bad and for example white text is wildly colored and hard to read.
I also tried different power supplies (USB chargers) and some power banks, but same results.

Here's a quick video I made:

It starts with PAL and then switches to NTSC in a loop.
For that I've used these commands which should be equivalent to mode 16 and 18:

tvservice -c "NTSC 4:3 p"
tvservice -c "PAL 4:3 p"

Is there anything that I can do about that or do I have to live with it?