If you're experiencing this, it's usually because you have a controller that is always pressing up or down, be it a physical controller or a virtual one. A very common culprit is older versions of vJoy. If you have vJoy, uninstall it or get a newer version.
If you aren't sure whether there is a controller active on the computer, you can open a list of connected controllers by pressing Windows key + R, typing joy.cpl and pressing Enter.
To remove a controller, either find a way to fully unplug or disable it from the computer, or, if you're logged into Parsec, go into Parsec's settings > Gamepad and click Unmap on the input that is stuck so that Parsec stops using it.