it could be the suspension setting. one thing i remember from the 3 across America was the issue w/ run flats. key note speaker was saying that the e90 was designed w/ the run flats in mind, so the suspension was designed and tuned softer to take away the harshness associated w/ the runflats. he went on to mention that chassis stifness improvement led to same ride characteristics w/ softer shock settings.
enough e90 comercial: my point is that, e60 shocks probably weren't designed w/ the runflats in mind. and problems that we are seeing could be attributed to that. hope you get your soultion, and that bmw would retune their shocks to work better w/ runflats. not having a spare must be great!