Check your agitator motor connection. How many wires does it have? Where are these wires connected?

Sounds like you've wired your agitator motor in such a way that when you turn on your heating element, your agitator motor takes the voltage from the heating element as well. So when you adjust your voltage on the agitator motor, it doesn't do anything because it's using the voltage from the heating element (you've connected it in parallel so your original connection of the agitator motor is bypassed so changing its voltage don't do anything).

It doesn't have anything to do with your home wiring. It's got to do with your brewing wiring - your control panel wiring.

Make sure your agitator motor is only connected to 1 circuit where it goes to 1 voltage source. Don't connect your agitator motor parallel with your heating element.