I'm running into the exact same issue as MajorJebus. It happens a few minutes after starting any game. I tried several things including removing the USB drivers and resintalling the components, switching the PCIe card from a 1x to an 8x slot, and switching between SteamVR Beta and non-Beta. Unplugging and replugging the USB cable between the battery to the wireless adapter on the headset brings the headset back online.