After doing the following, these issues have (finally!) been resolved:
I manually removed the Nvidia drivers, rebooted, and then did a 'custom', 'clean' install (tick the 'clean' button), and only installed the latest Nvidia graphics drivers (unticking the 3D stuff and Geforce Experience).
I uninstalled the UR824 audio interface and it's applications, unplugged all USB devices apart from the mouse, rebooted. Installed latest driver version, plugged in audio interface to USB, and turned on the device to finish the installation. Reconnected other USB devices.
Uninstalled 'Killer Suite' (downloaded latest driver only version first!), rebooted, installed driver only version of e2200 network adapter driver, rebooted.
Tested in gaming and in Cubase, no ASIO spikes and no audio drop-outs over a few hours of gaming and a couple of hours of fairly intensive audio work.
My gut feeling tells me that it was something corrupt with the Nvidia drivers, or possibly a conflict between the Nvidia Geforce Experience software and the Killer Suite.
One other thing that I changed was a PCIe setting in the BIOS that allowed you to set the affinity in the BIOS for different PCI generations. Mine was set to 'Gen3' as my card is a 'Gen3' PCIe videocard, but setting it to 'auto' (the other setting was 'Gen2') seemed to lessen the DPC latency spikes.
Hopefully this helps someone else.
