Fabio has been so helpful via email! I somehow managed to stop the crashing during idle, but now it is only the video service.
“The video service for decoding video stopped working. This may affect MediaBay Scanning, video import and playback. Please save your project and restart the application.” Cubase 8 and cubase 7.5 both say this message shortly after I start playback after returning from idle.
I wish I know what I did to fix it, but it’s possible one of these fixed it: I tried is the registry change to unpark all of my CPUs. Another is I uninstalled some programs. I also reset my paging file from a 200MB max to “Let Windows manage the size of the page file for me”