Based on your description, it seems there may be a boot timing issue in your system, and the motherboard is not initializing the SSD on time. Please check the following items:
- Make sure that that the SSD is connected to a PCIe 3.0 x4 slot.
- Check with ASUS support and make sure you are using the most recent BIOS (currently 2001)
- Review the Booting from an NVMe* PCIe* Intel SSD - Technology Brief, page 26-27. Here you will find the configuration recommended for your Motherboard.
Sorry, I should have included this.
I am running the ASUS 2001 BIOS (which is the latest)
I read the Intel document you refer to and have the CSM settings setup as detailed in the document.
I have made sure it's in a 3.0 slot and even switched it from auto to 3.0 in the BIOS, also it shows it running as X4 in the bios.
Not sure where to go next at this point, it boots maybe 1 out of 5 to 10 times. It's infuriating as once it's booted everything runs completely smoothly.