PS/2 mouse bug
In versions of Doom prior to v1.666, some PC configurations would cause the player to move erratically, including, but not limited to: spinning in place upon moving the mouse, firing at random intervals, or moving forward.
The cause was determined to be input related, and was fixed in later versions. Note that this bug is unrelated to one caused by the Logitech mouse driver, which is caused by the faulty driver itself. If you must use v1.2, it is recommended to use CuteMouse driver to remedy both errors.