SNES image jitters after 50/60 Hz mod installation

[Nekoi]

I installed a 50/60 Hz switch into my PAL SNES this weekend. Like on this website: http://www.gamesx.com/importmod/snes5060.htm

To be honest, I hate soldering and don't have much experience. This was the most difficult modification I've done so far. Still it seems to have been a success. The SNES still works and the switch does what it should.

But something's different than before. The image now jitters. Every second or so the whole image moves 1 Pixel to left then right back. Sometimes it is more frequent, sometimes less. It happens in both 50 Hz and 60 Hz.

I've connected the SNES to a normal CRT TV through a RGB Scart cable (this one https://www.retrogamingcables.co.uk/nin ... d-for-sale).

I don't have a second RGB cable for the SNES. But I tried a S-Video cable and a composite video cable. The image with these cables is stable. The image is also stable if I connect the SNES with the same RGB cable to my HDTV.

So the problem only shows up in that one situation, RGB on CRT TV. But of course that's the most important one.

Any ideas what could be causing it?