1998 Mitsubishi Legnum Car shaking at 50kms

Hi There, when I drive at 50 km and the RPMS are at 1500 the car start to shake.When I drive over 50 km it fine.When It cold it fine,but when it warms up it start shaking. I have changed the spark plugs.Taken it in to Mitsubushi.and They hook up there laptop and could not find any faults. Sorry to be a pain I got auto electrican to install a boost gauge. He told me that It is missing a vacum line from the regulator.I know it only Boost at 3 pound there no boost tap. Need your help or advise where I can take my car in to get fixed.

Thanks