I've experienced this issue while streaming Hollow Knight, Windows Store Version.
Since a regular game capture does not work with Windows Store games I'm using a Window Capture in WGC mode.
After roughly an hour the FPS for that capture randomly plummet down to about 10 FPS and stay that way until I disable the capture source for a few seconds, after which I it runs smoothly for roughly another hour.
I can't say I've experienced this behaviour before and it's limited to the source capturing the game while the rest remains smooth.
Since a regular game capture does not work with Windows Store games I'm using a Window Capture in WGC mode.
After roughly an hour the FPS for that capture randomly plummet down to about 10 FPS and stay that way until I disable the capture source for a few seconds, after which I it runs smoothly for roughly another hour.
I can't say I've experienced this behaviour before and it's limited to the source capturing the game while the rest remains smooth.