I have that common annoying issue. Just got Rift S, it wasn't working correctly at all when plugged directly to motherboard, but that's why I had Inatek PCIe USB 3.0 card left, after my pervious CV1. Now it works, once I have it going I can play for hours and it;s fine.
But when it's just resting on my desk, and I am not using VR, I keep getting this random disconnection of Display Port or USB, then all of the sudden comes back and that just keeps going. Sometimes to use it again, I have to unplug usb and plug it again.
My PCIe card is powered from PSU, but it sill seems like power issue, just ordered USB hub with 3A power supply, but kinda have a feeling that it won't help.
Not sure why I am writing here, as I know it is known issue with Rift S and I didn't find any solution. But maybe I am missing something ?