This is normal behavior if the AC Fan Speed **** is in the "AUTO" position (Or 1 rotation click from the off position). If you're saying the fan speed intermittently changes when using the manual speed selections, I suspect the problem is with the AC control unit in the dash (the board with all the control ***** on it). You can find these on ebay for relatively cheap.
-Icer

 

Welcome- Just a bit of detail to add to Icer5160's post. The "auto" setting is automatic temperature control. Using this will allow the fan to go up or down as needed to maintain a desired temp setting. This system may be operating in other settings if the HVAC control unit (a small rectangular white box behind the lower center stack *****) is malfunctioning. There's no real way to test this unit that I know of- you usually just replace it with another OEM unit and see if the symptoms go away.

 

Sometimes it is the waterfall not being installed perfectly or the radio is screwed in too tightly distorting the surrounding frame especially on aftermarket stereos. It will at times cause the AC to turn on randomly as well.