Having an intermittent problem with my steering and would welcome suggestions on resolution. Occasionally I get a vibration followed by a strong (one time unsafe) back and forth vibration on my steering wheel. Happens around 45 mph but if I slow down for a few minutes it goes away. Has happened maybe four times and have had it in to BMW twice and they cannot find anything wrong nor replicate; suspension is tight everywhere. Car has new tires and rebuilt CV shafts (by me) but I drove it about a 1,000 miles after this service before vibration started. Also has new rear lower control arms. Any thoughts? It's an '05 with about 250,000 miles, manual, well sorted. I've had it since almost new, bought as a CPO with 10,000 miles. Getting ready for my bimonthly SC to NJ home switch and wondering if I should take the other car. (That's a Porsche Cayenne GTS with a manual too. I prefer the X5 but my wife loves her Porsche.). Any thoughts or lines of investigation would be appreciated. I did not repeat the alignment or wheel balancing, that and a tire rotation are next.

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2005 X5 3.0, Manual Transmission, Black on Black, Sport Suspension, 250,000+ miles and counting, Best SUV I've ever owned