Why Does My Desktop Monitor Randomly Brighten or Darken Until I Move the Cursor?

This sounds more like an automatic display feature than a failing GPU. Check the monitor’s own settings for things such as dynamic contrast, auto brightness, eco mode, ambient-light adjustment, or a presence sensor. Also check the operating system’s display and power settings for adaptive brightness or content-based brightness controls. Since moving the cursor changes the behavior, the monitor may be reacting to inactivity rather than a graphics-card fault.

Try switching the power mode to a high-performance setting, although that probably won’t affect brightness if the setting is already at Best Performance. It’s also worth testing another display cable or input and updating the graphics driver. If the effect only happens on this monitor and not another one, the monitor’s firmware or internal brightness controls are more likely than the GPU.

If your monitor has a USB connection or a built-in proximity sensor, check whether it has a feature that dims the screen when you move away. Some displays also dim or brighten based on the average content on screen, especially when dark and bright windows are being opened. Disable those options temporarily to see whether the random changes stop.