Didn't resolve the issue for me. I turned off my Pebble, and forgot the device from my iPhone. Then the normal process happens:
1. Place a call
2. After 45-75 seconds (it varies), it starts to break up
3. I switch the call back to the phone
4. Call continues normally for ~30 seconds. I see the phone vanish from the center screen, and then reappear
5. Car steals call back
6. Return to 2.
I was parked this time around, but it's a lot of super-safe fun if this is all happening when driving, so now I never take the call via the car connection. It's still legal to drive without handsfree in Tx (it shouldn't be), but I personally hate doing it, and it looks really stupid in a car that's a technological marvel in every other way. I can recreate the problem with any of the phones in the house (iPhones 4 and 5), but the SC has never been able to recreate it :-/