Sounds to be dirty. There is a thing called tuner cleaner you can use to clean it. You will need to remove the control knob and try spraying into the control,but you may have to get inside to clean it up good by breaking down the unit.
Kitchen Aid suggests putting a dime in the bowl and turning the mixer on low. If the beater attachment pushes the dime around the bowl, the attachment is too low. If dime doesn't move, the attachment height is too high. When the attachment height is just right, the dime will only move a little.
Sounds as if the amplifier is overheating? Does it do this at all volume levels or only when turned up to higher levels- if so it's likely the amplifier inside of the receiver.
If you do not know what you are doing it is better to throw it away and get a new one. You could get electrocuted if you try yourself. Probably cost more than a new one to get someone to repair it.
Not sure how to change the switch but some knobs are retained by a small grub-screw or a spring clip and it is sometimes worthwhile checking the switch shaft is actually being turned by the knob.