That could be a big part of your issue there.
It seems your TV doesnt support 640×480 and wont natively display 720×480 but rather is internally scaling 720×480….which it doesn’t sound like it’s doing very well. Have your tried line2x? You might get lucky and hit a native resolution in your TV at 1280×960 or 1440×960.
The user guide and spec sheet for your TV aren’t very helpful. Just say max res of 1920×1080 and ntsc. But it is 120 hz. Anybody have any opinions in whether 120hz would create any potential for not playing nice with the ossc? I would assume no, but thought it worth asking.