A lot of people have been having issues since the last patch. My FPS drop was random as well, Hedge said their was no patch when my game started acting up. But I definitely had a update in the bottom of steam.
Mine has been happening for almost a month now. Last reply hedge is trying to get some of the engine Devs to look into it. These issues have been going on for a long time, and seem to happen at random. My wife had the same issues I’m having for around a month as well, then the Summer Solstice patched fixed it for her.