A multisync monitor isn't a flicker fixer on its own. Its advantage is that it can show TV freq (15kHz) signals, and VGA (30kHz+) signals.
So if you throw an interlaced mode at it, it's still going to flicker. But the advantage is, normal (S)VGA monitors can't view these modes, so you can't play old games on them without a scandoubler.
Try DblPAL, Productivity and Euro modes. The monitor drivers should be on your workbench disks or in SYS:Storage/Monitors. Just drag them to SYS:Devs/Monitors, reboot, and select one of the new modes for workbench.