Shaking image on SNES core only on VGA output

[bootsector]

I've just got a brand new 17" CRT monitor. I'm using scaler only for HDMI and it's set for 1080p, so I'm using forced_scandoubler=1 for the VGA output. Everything looks awesome on NES, Genesis and TG16 cores, but I get a shaking image (top part only, the bottom looks great) in the SNES core.

SNES core looks great on 480p on this monitor if I enable vga_scaler=1 and set video_mode=6, but I didn't want to use the scaler for VGA output.

What could be making the SNES core to make the image shaking at the top? Maybe the SNES vertical resolution? Is there a workaround for it if I wanna use barebones forced_scandoubler=1 on SNES core with this monitor?