Only works speeds 6-8. Turns off when moving knob to lower speeds. Tightening two screws in the back of the mixer above the power cord changed the working range from 4-6. Any ideas?

Obtain a copy of the Repair Part List for your model from Kitchenaid.com.
Download a Copy of the Kitchenaid Service manual.

http://www.kitchenaidparts.be/centers/servicemanual/mixer.pdf

You need to adjust the mixer speeds:

Remove the screw holding the End Cover and locate the three screws from the Control Plate.
All three (2 top left and right, and 1 bottom) must be adjusted.

1. Loosen the lock nuts enough & Turn the 2 top screws clockwise until they cannot be turned
anymore. Leave the bottom screw alone for now.
2. Use a dab of white paint or paper whiteout to mark the positions of the Phillips screws.
3. Plug in your mixer and Set the speed selector to "Stir".
4. Turn each top screw One full turn couterclockwise (alternating between Left and Right)
Keep making one full turn at the time (for both left and right) until the motor starts running.
5. Put your hand near the Agitator shaft and allow it to brush your finger slightly. Use your
watch or clock and start counting the number of times the shaft touches your finger in
60 seconds. The correct count should be 60 RPM (Rounds Per Minute). If it is less
or more than 60, give each screw 1/4 turn counterclockwise or clockwise and start counting
again. Now tighten both lock nuts.
6. Set the speed selector to "6" and turn the bottom screw either clockwise or counterclockwise
to achieve a speed of 180 RPM.

For your reference, the planetary RPM's for the 5-qt. tilt-head Artisan Stand Mixer are as follows:
Stir speed - 60
Speed 2 - 95
Speed 4 - 135
Speed 6 - 180
Speed 8 - 225
Speed 10 - 280