In case it can be useful to someone else, my problem was the monitor rate (set to its default: 60Hz).
Missed it entirely in my first search, but the explanation was right here
https://fs-uae.net/perfectly-smooth-scrolling
All I had to do was create a profile for Amiga Forever in the Radeon Settings with Frame Rate Target Control set to 50Hz.
Took 10 seconds, runs great now!
P.S. Have no idea why AF2014 works fine on Win 7 as it's the same PC and monitor, but both versions run great, so it's a 'who cares?' matter I guess!