Oh that makes sense. My 4k set has that sync issue at 2x in 60Hz, but it displays at 3x via 1366×768. It’s that exact shimmering effect I was referring to and I don’t recall noticing it on a CRT, so that makes perfect sense.
I might play around with it on my smaller TV which accepts those modes, as my 4k set is too fussy to allow the headroom for messing around. Some day I’ll just buy an LG OLED and all my problems will be solved. 😛