The first thing you need to do is remove the end cover on the mixer, there is a screw at the top you need to undo then in an upwards motion you should be able to pull that off. behind there is the speed control plate. there are two screws at the top, and one at the bottom, the two at the top need to be adjusted the same amount, to speed up the mixer you turn the screws out, to slow down, you turn them in. you want to do each screw 1/4 turn, then test the mixer, see if this will slow the mixer down, so it regains the other speeds. If this adjustment does not work, then the problem will be the phase control board, that is part number W10325124, we sell the board, here is a link to that on our website: http://www.mendingshed.com/kaphaseboard.html.
Also, here is a link to the service manual for your mixer, it will go into more detail on taking the back cover off, and adjusting the speed control plate. Also if you do end up needing to replace the phase board it will show you how.
Service Manual: http://ep.yimg.com/ty/cdn/mendingshed/kitchenaidservicemanual.pdf
If you have any questions feel free to call us.
The Mending Shed