kinda looks like this (0:26 for reference): I guess try to set the framerate to 60?

Long story short: It can't handle composite > scart, only rgb scart.

If you only need to capture the Wii, component cables are really cheap and the quality is way better (LGP supports component too).

If you need to capture other consoles with composite cables, get a dazzle or gv-usb2. You can use composite to HDMI converters, but those usually produce a darker image. Which isn't a problem if you don't play dark games though.

I don't have a LGP but it works fine on the LGX. Just remember only PAL gamecubes support RGB.

Change Port! gamecubes support RGB.