In the Artisan mixers the problem can be caused by two different parts. First the speed control plate. When you remove the end cover from the mixer the first thing you see is the speed control plate. There are 3 screws on this, two at the top, one on the bottom. Try adjusting the top two screws, to adjust, turn in the control plate adjusting the screws an equal amount.
Also check to see if the control plate spring is unhooked. Remove end cover, and check control plate spring. If unhooked, attach to bottom of the control plate and squeeze the spring end.
It can also be caused by incorrect wiring, check the wiring, the last page of the service manual will be a wiring diagram for your mixer. Here is a link to the service manual. http://lib.store.yahoo.net/lib/mendingshed/kitchenaidservicemanual.pdf
Last, if the control plate is properly adjusted, the control plate spring is hooked correctly and the wiring is correct you will need to replace the speed control phase board. With the line cord disconnected, pull the t-bar contacts to open and insert a piece of paper between them so they cannot close. Reapply power-the mixer should just run or buzz trying to run. If it operates at full speed, the phase control is defective and needs to be replaced.