You don't need to buy one, you need to check it out and properly diagnose the problem. It would be silly to replace it if it only needed adjustment. The fact that it works sometimes suggests it isn't broken, just in need of adjustment.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue