Why is AC not blowing on the correct speeds?

It originally started with my AC only working on Low which makes it blow high...speeds Two and Three were pick and choose it depends on how you cycle through the speed settings to get the actual speed that actually blows, but the highest speed was always off. Recently no matter what speed setting is chosen the ac always blows high and won't turn off unless i set the speed on high and give the knob a quick tap which sometimes comes right back on until i tap it again. Im aware of the blower motor resistor, but was maybe a control switch behind the knob. Any suggestions?