Sadly I don’t have anything else to test with VGA and the AV3 input. (Like the dreamcast)
I have the SNES running right now through a good scart from retrogamingcables UK into AV1 and it does still randomly cut out.
In the last 30 min of running it, it has only cut out once, for about 5 seconds them slowly came back.
I’m just using the default settings on AV1./