i have similar problem, but BT was working more or less fine. Only hang up few times. But main problem is that i could not use BT in my car any more. Phone connecting to car with no problem, and i could receive calls from anybody, but as only i try to call from phone, not from car HF system, i get problems. While phone dialing, it happens in background mode, car audio system working, but as only recipient answer the call, Gnex turning his internal speaker, but not the car's HF system, like it was on 4.0.4 - 4.1.2. I flashed custom rom - the same problem, then i download and flash stock rom again, and nothing happens. Does it helps if i now reflesh system.img
I asking because i gave Gnex to the warranty service (with all above mentioned explanation) and they gave me back Gnex with 4.0.4 and claim them for that. now i don't want to flash 4.2.1 until they response.