I seem to remember a discussion with a Volvo mechanic saying they provide motor mounts that resonate at different harmonics. Normally the installed version is designed to dampen vibrations at the near idle speed, the alternates move up higher on the rpm level.

The reason I ask is I wonder if you should just be turning the idle speed up a slight amount and forget it. You have mentioned that touching the gas ever so slightly makes the vibration all but non-existant. Could your tach be reading slightly high?