Free space on hard drive going up and down too much
Install Homebrew (www.brew.sh)
In the teminal, use this command to install ncdu

brew install ncdu


then, run it

cd ~; sudo ncdu


You will get a nice overview of the size of all folders in your home directory, largest topmost. It should be easy to track your gigabytes.

If this does not give results, you can run ncdu for your whole disc as well. Just go to /Volumes/DISKNAME e.g.

cd /Volumes/MACBOOT; sudo ncdu


You should also make sure that no application is simply "eating ram" by running

top -o mem


In the top lines, if there are applications that have a "G" in the "mem" column, they use gigabytes of ram. Make sure this is adequate. You might have an application with a memory leak installed.