Ford Focus 05 2.0 eng 109 thou miles vibration prob in steering wheel at idle?

Ok this is a flaw with most ford focuses but the degree of vibration differs at different times. One time no noticeable vibration the next more. There's no noticeable vibration when I drive goes away as soon as I hit the gas I don't think it is the rotors. Vibration continues in steering wheel when I let off brake at spot light only goes away when I drive. I'd guess motor mount but why would the vibration differ at various stops. I'd say tune up but car drives fine no vibration someone suggested jacking up engine a wee bit under oil pan and see if vibration goes away. I'm guessing perhaps an air leak in one of the hoses going to the engine. What do you think

Is your idle dropping (Tach drop below 500rpm) when this happens? I had a 2000 that would stumble at idle (drop rpm) bur run fine at speed and no gas mileage losses - changing spark plugs helped the first time but 20k miles later it was doing it again - I eventually chased it down to a cracked coil pack (new one at Amazon for about 25usd fixed the problem forever).