Bluetooth Car Disconnects :( Having a pretty irritating issue with my bluetooth connection and my car. The phone will connect up no problem, but as soon as you try to play a track, switch apps or sometimes even unlock the phone, it'll say the device has disconnected, with a few caveats.

Details;


Device is a Nexus 4 (The LG One), Rooted running an official rom of 5.1.1, i haven't run any updates or updated any apps in the past week since this issue cropped up randomly one morning.
Car is a Ford Fiesta, built in 2015, UK 15 plate.
I'm connecting my phone to the built in Ford SYNC system, had no issues with it before on the same device


What i've tried; 


Full factory reset on phone
Installed same ROM again from google
Full Dalvik cache wipe
Full factory reset via CWM recovery and via TWRP before flashing roms
(Separate occasions)
"Factory Reset" in car
"Master Reset" in car (that cleared all my audio preferences for the radio)
Remove pairing from car and phone and repair (connecting from phone first)
Remove pairing from car and phone and repair (connecting from car first)
Allowing car to access contacts, messages and phone audio
Allowing car to access nothing but media audio


I'm truly at my wit's end here it's driving me absolutely insane, and it only started randomly last week.

What's weird, is that my phone will connect up no problem, the SYNC system on the car will tell me my phone's connected, but as soon as i go to play music, switch apps or unlock my phone, it'll tell me it's disconnected. What's REALLY weird though is that my car will still give me a reading of my phone's signal strength and battery level, even when it's apparently disconnected, and it seems pretty accurate as well.

Just thought i'd run this by you guys incase there's something blatantly obvious i've missed before i head into the Ford garage and start whining at them
So an update for anyone who's stumbled across this question in future;

I think I've managed to solve the problem. Connection restored and music is playing no problem, I've yet to test phone calls and texts yet though.

It would appear the problem was with SYNC and my text messages, I was using the Google hangouts app previously which is notoriously unreliable on older handsets, installed Google Messenger (their new SMS client) this morning, just tried connecting again during lunch and all seems well and stable.

If you're having disconnects especially with a Nexus 4 and (by chance) Ford SYNC, try force stopping, uninstalling updates and ultimately disabling Google hangouts. I'm going to fully remove the app tonight via ADB but for now the disabled state seems fine.