@Jonlan74
When you connect the Lumia 950 XL to your PC via USB check device manager. Look to see if it ever makes it to the Lumia boot manager. If that listing never shows up in device manager then look for Qualcomm HS-USB QDLoader 9008 under Ports (COM).
It is possible there is a hardware fault but if there is not and it is botting to either the Lumia boot manager or QC 9008 mode then it could be a software or boot loader issue the requires a more in depth recovery then a standard hard reset.
The pic I posted here is of a Lumia 929 that is doing this. It reboots every 30 sec or so, with no vibration and goes straight into QC 9008 mode. I know for sure it is a hardware issue because prior to doing this it was dropped. When attempting to recover it using Emergency flash loaders it is able to boot, some times reflashing it fails though because certain memory segments are unreadable. I can using Windows Phone Internals reflash it with the original FFU file but it never boots.
So try not to give up until you are certain it is a hardware issue or you just want to move on from it.