My first 530d with active steering and runflats had a vague steering feel at high speed. It wouldn't hold a straight line without constant correction and seemed to be affected by the slightest side wind. My 535d does non of this. Its almost as good as my old E39 just a shame about the wind noise.

 

I would sugest it's your run flat's, mine used to do this for the first few thousand miles untill they had bedded in. Also alignment could well be out, mine car was out from new as was a chap's from work who also has an E60. If it doesn't get any better ina few thousand miles then take it back for a KDS alignment, they should do it free of charge. Tyre pressures can make a big difference, I have tried running a few PSi over the recomended 29 front and 35 rear, but it made the ride to hard (handled a bit more positive though) so I have I settled for the recommended pressures and she handles and rides just fine. Also when the KDS was done they set the fronts with only 0deg 2mins of toe in, this made it more responsive to turn in

 

I'd check out tyre pressures and do all the rest first but I noticed that my E60 is slighly more prone to cross winds than any of my rock solid E39s I've owned. Obvious check is to drive down the same road under different wind conditions. Either way we are talking very a mild effect but after the E39 I found this disconcerting though soon got used to it.

 

I would agree with the above:
(a) runflats are bad for tramlining - real tyres much better
(b) UK motorways seem to have developed deep ruts in nearside lanes - particularly over the past couple of years (heavier lorries?)

Boy you can get some interesting moments when towing a high centre of gravity trailer when the trailer suddenly catches in these ruts - not a danger things, but certainly causes a frisson !

 

Ninety percent of my 550's tramlining issues were resolved when I changed from runflat Dunlops to Michelin PS 2's. In addition to this the steering feel improved 100%. So if you are happy now just think how good it could be in the future if you get rid of the runflats.