I have had numerous instances of the Bluetooth connectivity between my “HTC Desire” and the in-built car phone system dropping out.   My phone was normally left with both Bluetooth and wireless switched on for general use at home.   I have found that switching off the phones wireless capability when in the car ‘fixes’ the dropping out issue, and I now have a continuous and uninterrupted  connection when driving.    Hope this snippet is of use to other members.