Don’t know if any of this will help, but you might try the following:
1. Restore all brush variants in the currently loaded brush library to their default settings:
Brush Selector Bar menu > Restore All Default Variants.
2. Reset your Brush Tracking using lighter than normal hand pressure:
Edit > Preferences > Brush Tracking
If that doesn’t help, try resetting your Brush Tracking again, using even lighter pressure. If you find the right settings, take note of what they are so you can return to them later.
3. Open Painter 8, restore all brush variants in the Painter Brushes default brush library to their default settings, then save the Digital Airbrush variant giving it a new name: “P8 Digital Airbrush.”
4.Close Painter 8, go to your file manager, and copy the P8 Digital Airbrush then paste it into the following Painter IX folder:
Corel > Corel Painter IX > Brushes > Painter Brushes > Airbrushes
5. Launch Painter IX and with all brush variants restored to their default settings, open a new Canvas and on the left side, test the default Painter IX Airbrushes’ Digital Airbrush variant. On the right side, test the “P8 Digital Airbrush” variant and see if there’s any improvement. Use the same colors and the same hand pressure for a good comparison.