2006 Sonata 2.4L Strange Rough Running

My 2006 4cyl 2.4L Sonata has developed a
very strange fault. It idles fine, around town
it's fine, cold or hot & it still has plenty of
power.

BUT, when cruising along the freeway once
fully warmed up it begins sporadically
running rough. If I floor it it gets better. If I
slow down it gets better.

I've checked the coils & there's no sign of
flash over or corrosion on the connectors. I
did replace the plugs, they were old &
needed to be replaced soon anyway & that
improved the matter, but it's still there.

My obd2 code reader shows nothing.

I'm really confused, everything points to a
fault coil pack but which one & why does it
get better when I accelerate when, in theiry,
it should get much worse?

I'm stumped, all help will be greatly
appreciated.