Quick question, you say one big session. I use sessions also, but open raws by folder, i.e. each shoot is in a separate folder so C1 creates a new cache folder for each location. Each time you point to a new folder C1 will create the preview cache info in a "capture one" folder under that same location. Are you putting all your files in one single location? and then using C1? That may be an issue for a session type work flow not sure.
I keep raws on Mac or PC in separate folders, First by camera, then by date. But this can also be used in a catalog workflow to.
You may also need to just totally uninstall the app, and reload it. P1 has instructions for both Mac and PC, as by just un-installing the app, you won't remove all the instances of it in the registry, (on win). Mac also has a few things that need to be checked.
Paul Caldwell