IPhone :: Bluetooth Devices Can't Reconnect To 4 After Some Time

so I've had these Bluetooth issues since I updated my iPhone 4 to iOS 5. I've searched and searched and found nothing, so finally I decided to post my story here. First of all I'd like to point out: I've tried everything! ..but more on this below. I have a Bluetooth capable car stereo, the Sony BT2600, in my car. It works like a charm and I've been using it for listening to music via A2DP (Bluetooth Stereo Audio) since I updated my HTC Prophet to Windows Mobile 6 back in 2008. Since 2009 I've been using it with an iPhone 3G, then a 3GS, and finally with my iPhone 4, which I've owned since october 2010.Calling worked, listening to music worked, and since iOS 3, I think, AVRCP has been working fine, too.

Now to the things that worked until before the iOS 5 update:When I got into my car, turned on the ignition, the car stereo automatically tries to connect to the last connected device, so unless BT was disabled on the iPhone, it connected without problems and I could start music playback by pressing the Play/Pause button on the unit, without having to pull the iPhone out of my pocket.When I got out of my car, turned the ignition, and thus, the car stereo, off, the iPhone would of course disconnect.Here's where it becomes interesting: When I would go for another drive in my car, the head unit would reconnect to the iPhone, just as in the first point of this list.Same when starting the car's engine, which cuts the power to the head unit for one second. It would automatically reconnect to the phone and I could press the Play/Pause button twice to resume playback.Furthermore, when Repeat was not enabled on the iPhone's iPod application and the current album / playlist / podcast would end, the Bluetooth connection would remain active, which was good, because I would still be able to receive calls using the hands-free functionality of the Sony head unit, and I could restart playback by pressing the Play/Pause button on the latter.My Apple aluminum Wireless Keyboard would also connect flawlessly whenever BT was enabled on the iPhone.Also the Bluetooth tethering feature would work fine - I could just connect to it via my MacBook's Bluetooth menu, without having to do anything with the iPhone.

Since I updated to iOS 5,my car stereo doesn't connect to the iPhone unless it's been less than a minute that I'd enabled Bluetooth on the iPhone.same with the keyboard.When music playback stops (at the end of an album/playlist/podcast, with Repeat disabled), the whole Bluetooth connection is instantly lost, and can't be regained until I disable and re-enable Bluetooth on the iPhone. Which means that I can't take calls in my car after listening to an album or podcast or playlist when I forget to enable Repeat.Whenever I have to re-start my car's engine, the connection is, in more than half of the cases, lost until disabling and re-enabling BT on the iPhone.Same with tethering.

What I've tried so far:deleted all BT devices from the iPhone's list, re-paired - nothingrebooted, many, many times - nothingrestored to iOS 5.0.1 - nothinghit "Reset Network Preferences" - nothing, the BT devices even stay in the listhit "Delete All Settings" on the iPhone - nothing, BT device list still the samerestored to 5.0.1 again - nothingrestored to iOS 5.1 - nothingrestored to iOS 5.1 from DFU mode, setting up the iPhone "as new iPhone". Big step, PITA, but still - nothing. At least this has cleared the BT device list. But the issues remain.

Info:
iPhone 4, iOS 5.1