RE: XPS 15 9550 freezes randomly (on any OS)
This appears to be a BIOS problem. The 1.1.19 BIOS seems to have issues causing the machine to freeze when running normally. I downgraded to 1.1.15 and now seem to be stable (for this issue). I do have a Thunderbolt dock and had to have the Dock plugged in for it to keep the Thunderbolt port properly enabled. There seems to be a problem with a component(s) on the motherboard and the BIOS. The quality testing of the BIOS failed us here. After installed 1.1.15, I did end up reinstalling Windows since it had corruption from the crashes.
I hope this helps!