I get that same problem with my XM. There are points in town when if I drop down a small hill it just stops altogether for blocks. I wish there was a better place to locate the antenna ...

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2007 328i Sport, Monaco Blue
2011 X35D - sold -Space Grey
2004 X5 4.4i - sold - Kalahari Beige
2001 X5 3.0i - sold - Pearl Beige
2006 325i Sport, Sonora Beige
2000 323i - sold - Sienna Red