Trying lowering the output of the WiFi card to 75%. It fixed the problems for me. I know this sounds crazy, but it works. I hope this works for you. I can explain it later.
Actually this did not fix it for me.
Since my last post Dell has replaced the main board in the XPS13. This effectively made no difference whatsoever.
My test VGA monitor is a Samsung SyncMaster E1920 (I don't actually use this monitor but it exhibits the same problem I encounter with most VGA data projectors I find out on the road). This monitor is not detected via the Dell DA-200 at all. The monitor even when connected has a message "check signal cable".
If I switch out the DA200 and use a CableCreation USB-C to VGA dongle the "check signal cable" message disappears even before I plug the dongle into my USB-C port. The VGA monitor will work with the CableCreation USB-C to VGA dongle but the screen turns off and on every 10-20 seconds. Following a suggestion on another thread I disabled bluetooth. This fixed the turning off and on issue with the CableCreation USB-C to VGA dongle. The DA200 still does not work. I have tried it with bluetooth disabled and the the power output of the wifi card set to 75%.
Strange...worked for like 90% of people having the issue. Make sure your bios is up to date (1.4.4 - release notes show it has supposedly fixed USB-C stability) and update to the new thunderbolt software and then firmware. Update the DRIVER first, then the FIRMWARE.
Just as yet another troubleshooting step, try a different VGA cable if possible.
did you ever find a solution to this? I'm having the same issue. All drives, and firmware are up-to-date. Monitor, and VGA cable works on my other laptop.