Perret81,
I had the same problem (smartphone HUAWEI Honor 7), but at the end I found this troubleshooting:
1. use only original or high quality cable
2. in your phone, go to the setting and then open applications panel. Select Android Auto and delete the cache.
3. repeat the same passage also for Google Service.
4. open the GPS settings and select "only device" (this is required to reduce the electricity consumption which might cause the repeated disconnection you had).
5. if you have a Xiami device, remember to change the setting for the double applications.
I hope that help you out.
cheers