I think that we have seen this questions asked a few times already within the past few weeks. I think you might be having power issues due to a weak battery. I think from a 04 to a 06 model, depending on driving pattern, it's about time for the battery to be weakening, so might be time for replacement.
2012 650-xDrive Convertible. Titanium Silver Metallic - M Package - Driver assist package- Cold weather package - luxury seating package - Bang & Oulfsen system.
2008 650 Convertible 6 speed manual
2004 645 Convertible 6 speed manual
1994 540 6 speed manual