2002 Chevy Venture Heater Issues

I have an '02 Venture, the heater only works on high settings. I have changed the Blower Motor Resistor and still have the same problem. Also the heat will randomly change blowing speeds while driving and sometimes kicks out if you change the controls(usually when switching to a lower speed.) Any advice would be greatly appreciated.

if you replaced the resistor then you either got a bad [new]resistor ,i would take it back and get another one or the control is bad.one other thing to check is make sure the ground wire to the blower is not broken or loose