The freezer section of my refrigerator flucuates in temperature. It can go from 0 degrees to 4 - 6 degrees. What causes this?

probably a thermostat for the freezer
have a fridge technician check it out

I agree with your analysis, and recommend that you replace that fan between the two sections.

refrigerator dial settings work from hot + 5 degrees (5) to real cold (1) 0 degrees and in the freezer section they work in the opposite direction (1) is cool - is minus 5 degrees and ( 5) is minus 25 degrees
those figures are example figures to show how the temperature in the units will be in relation to the dial numbers

UR COLD CONTROL THERMOSTAT, UR AIR FLOW VENT, UR THERMISTORS, A EVAPORATOR FAN NOT WORKING OR OBSTRUCTED

FREEZER /FRIDGE NOT COOLING: Optimum refrigerator temperature at or below40° F(4° C). The freezer temperature should be0° F(-18° C). Check temperatures periodically. Appliance thermometers are the best way of knowing these

The freezer temp should be between 0 and 5 degrees and the refrigerator section from 34 - 37 degrees.

Nominal tempretures for the refrigerator side are 36-38 degrees. Nominalk tempreture for the freezer section is 0-4 degrees. Your main board that controls your entire refrigeration can be found on this diagram as 1-5 http://c.searspartsdirect.com/lis_png/PLDM/50039314-00004.png
What tempreture are you trying to achieve? By setting the unit colder you also cause the unit to work harder as well shortening the lifespan of your refrigeration compressor as well as other key parts that perform to make your unit produce cold. Some key things to look for are any type of freeze up on the rear wall of the freezer which is termed as a defrost issue and if you do have this issue message me back and I can instruct you to solve this situation. I await your reply
Thank you,
Rick

A refrigerator should maintain temperatures at or below 40 degrees F. The freezer temperature should be 0 degrees F or lower

Actually, it's not as you think. Ice cubes are made between 0-4 degree Celsius and ice creams are frozen below -3 degree Celsius. Your freezer temperature may be between 0 and -3 degree Celsius and that is causing the problem. An ideal freezer should have temperature between -3 and -30 degree Celsius.

between 34 and 38 degrees Fahrenheit.