My first post on here but been lurking for a while

I recently bought a 40d M Sport the car is a 2012 ex demo and has the 20" 333 M Alloy wheels with run flats. When we test drove a car at the dealer we fell in love with the drive as it was "tight and gripping" - that car too was on the 20" tyres and same alloys but we didn't buy that as it never had the spec we wanted.
We eventually bought a car from a dealer at the other end of the country without test driving it and had the car delivered to us as we got a smashing deal. Everything is good with the car apart from the ride which is too bouncy over uneven road surfaces. It floats and feels very loose - almost like a suspension issue. Spoke to the selling dealer who was very supportive and told me to take the car to my local dealer for an inspection. I have now taken the car to two local dealers who have told me there is nothing wrong with the ride, tyres or suspension and that's the way this car handles with 20" tyres.
I swear the car we test drove was not like this so I test drove another X5 at the dealers with the 20" wheels and it felt pretty similar to mine - i.e woblly on uneven road surfaces.
So my question to you folks - would changing to non run flats help or has my car got issues that the dealers are unable to diagnose? If I'm doing 50 mph and I hit an uneven road I feel like im being thrown around the car!
The cars I test drove don't have 7 seats but mine does - now im beginning to wonder if this has something to do with the self levelling rear axle option on the 7 seaters?