Okay, I believe I have finally got this resolved fully for myself, I've tried everything to replicate the issues after trying this fix and so far it is holding up for me. This fix has a caveat though, you will lose Handoff - at least until Apple can fix this.
The steps are:
- Check on your iPhone under Bluetooth devices for any iOS devices running iOS 8 or Macs running Yosemite. These devices will be listed automatically and if you select the ! icon, the option to ' Forget this device' will be greyed out. Take a note of these devices.
- Unpair your Bluetooth connection from your car.
- Turn off 'Handoff' on your iPhone - Settings \ General \ Handoff & Suggested Apps \ Handoff
- Go to each of the devices in turn from the first step and turn off Handoff. iOS - Settings \ General \ Handoff & Suggested Apps \ Handoff. Mac - Settings \ General \ 'Allow Handoff between this Mac and your iCloud devices".
- Now select 'Reset All Settings' on your iPhone under Settings \ General \ Reset.
- After the iPhone restarts, turn Bluetooth back on. The devices from step 1 should no longer be showing.
- Pair your iPhone with your car. All should be working fine now.
The above will continue to work provided you leave the 'Handoff' option disabled on all your devices. The 'Reset Networking' option did not work, there must be some setting stored which is only affected by the 'Reset all settings'. This means you will need to do some device configuration again after the reset such as wallpaper, sounds and other personalisations but this should not take long to put back.