I had same problem.. luckily it was still under warranty. They found that the rear right sensor was faulty and replaced.
The last few weeks its been working perfectly!! .. for how long.. who knows

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3.0D Sport Auto, CARBON BLACK! Grey Leather, Smokey Grey Trim, Running boards, auto dim folding mirrors. 6 Disc CD Changer. Tinted windows. 19" style 132 Wheels. PDC.
Sports Pack. 6 Speed Steptronic. Communications Pack - SATNav MKIV, Digital Television tuner with Teletext.