Running out of options
Small grid spacing and snap to grid enabled might be the problem.
Make sure brush dynamics is set to off.
Can you post a screenshot of the 'sawtooth' line and including tool options in force.
Uninstalling Gimp rarely helps. All the settings are stored in your Gimp profile ~/.config/GIMP/2.10 and the problem is probably there.
Small grid spacing and snap to grid enabled might be the problem.
Make sure brush dynamics is set to off.
Can you post a screenshot of the 'sawtooth' line and including tool options in force.
Uninstalling Gimp rarely helps. All the settings are stored in your Gimp profile ~/.config/GIMP/2.10 and the problem is probably there.