I had the exact same problem. I've had my 2018 for about two weeks and it was happening at least once per drive. I did a bunch of experimenting and I think I finally have it squared away, but I did multiple things so your results may be different than mine. Here's is what I found:

• I have a Samsung Galaxy S8. Samsung's default power management features are a little overly aggressive and were definitely causing the bulk of my disconnects. I put in power management exceptions for my driving apps (Android Auto, Waze, Audible and Amazon Music) which reduced my disconnects substantially.
  
• Next, I found that when a disconnect occurred, I would have to unplug and wait about 30 seconds before I could reconnect. I later discovered that if I unplugged/plugged the phone in twice it would reconnect and launch Android Auto immediately. I'm not sure why, but that was my stop-gap solution for a little while.
  
• Finally, I started to suspect my USB-C cable. I ordered a new shorter cable from Amazon and I have not had a single disconnect since (only four days ago). So at least in my case it looks like the new cable made a big difference. This is the cable I am using.
  

Anyhow, all of that may be totally coincidental and I'll start having crashes again next week, but maybe it will be of some help to you.