300 SD Vibrating Like Washing Machine-PLEASE HELP!

[regsatx]

I've driven my "new" 300SD about 600 miles now and I can't figure out what is causing a distressing but intermitten problem. The car, which I bought with 224k miles, sometimes VIBRATES at idle. It seems to be getting more intense. However, it does not do this all the time??? That's very confusing.

A couple of days after purchasing the car, I noticed this. I was advised that the brake discs might be warped, since I noticed it when braking, just before the car came to a complete stop.

I've had a brake job since then (new pads all around and rotors resurfaced). The problem did not re-appear for several days and I thought it was over. Today, when coming to a stop coming off a freeway ramp, it was vibrating again!! Today I noticed that even after coming to a complete stop, it was still swaying like an old washing machine...I mean you can see the hood star moving! When I put vehicle in PARK or NEUTRAL, it smoothed out. Idle speed is fine, even when vibrating, at around 850 or so rpm.

So what gives guys? What could cause this to happen SOMETIMES but not ALWAYS?

Another dissappointment: filled up today after driving 157 miles, and car took 9.5 gallons to fill up; THAT'S ONLY 16.5 MPG. Where's my diesel economy? This is confusing because when it's not vibrating, it runs extremely well and is a very clean car that's been well maintained???