Thanks Calamity, with waitvsync at least I came back to previous situation, with 100% speed on 99% of the play time with very occasional single frame rate drops to 97%. But just to give an indea, it happens 3 times across 3 minutes of game play. As told, if it wasn't for sound slowing down, probably I wouldn't even realize. But given the music slowdown it's a bit annoying.
This doesn't happen without frame delay.
@R-typer It doesn't seem a performance issue, given I get the same result with my selection of '80-'90 favorites with frame delay set from 5 to 8. With 9 generally I start getting consistent slow down. Below 5, games are too fast. From what I understood this is the expected behavior using a PC CRT 70-120hz monitor.
Thanks, regards