These issues are well known with Elgato devices. Any modern to-spec HDMI console usually works without a hitch, but any user mod or scaler - the OSSC, the RetroTink devices, internal HDMI mods like DCDigital and WiiDual - produce footage with stuttering or audio sync issues. I used to own an Elgato HD60 and a HD60 Pro, and I experienced these issues firsthand. I use the 4K60 S+ for some things (it's super handy to have a capture card that doesn't require a host PC) but it still doesn't play nicely with mods and scalers.
If you want a capture device that plays nicely with the RetroTink and such, I'd recommend the
AverMedia Live Gamer 4K, which - aside from some occasional audio issues with PlayStation 3 streams - has handled everything I've thrown at it: every BlackDogTech/PixelFX HDMI mod, the MiSTer, the OSSC, RetroTink-5X Pro... the works. It will also capture 4K at 60FPS with passthrough, which would be an upgrade from what you've got now. If you need something external, I've had similar success with the
Magewell USB Capture HDMI Gen 2, but it's more expensive and less user friendly than the Live Gamer 4K.
If you're interested in the Magewell device, I'll sell one to you for half of the Amazon price. (And I swear I'm not just recommending it because I'm trying to get rid of one.) These are most definitely not the only options out there that worked; the Shmups Hardware forums can probably recommend some other devices, and they may be cheaper (<$200) than what I listed here.
