Apple Watch affecting car kit connectivity

Since the purchase of my Apple Watch in April 2016, I've continually had problems with my iPhone 6 connecting to my Nissan bluetooth system. Phone connects, but calls drop off, conversations break up....very frustrating! I tried all the usual things, reset all 3 devices back to factory settings, car kit, iPhone and watch with no luck. BTW.....there are no other bluetooth devices connected to the phone. Was resided to the fact that every time I got in the car, I had to remember to turn off the watch or put into flight mode. So I was excited to get my iPhone X and put this problem behind me...but I was wrong, experiencing the same problems.

Reading some posts on forums, this problem has been around for some time, but no one has really offered any solutions. Is anyone able to offer any advice? Would be extremely grateful.