My Kitchenaid K5SS 300 watt standing mixer launches into the highest speed regardless of the speed selection. It functions with a sliding knob and I can barely ease it from off to stir and it goes from no power to flinging flour all over the place. suggestions? When move speed control it is on high. Just started doing this. It is about 5 years old but not used much. Dale

I would recommend that you first check the Hall effect sensor which determines the speed.  I had this exact problem (but mine occurred with a 600 professional model) and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My blender worked as good as new! But I was curious to know if my original sensor was defective so I put it back in mixer and again made sure all of the connections were tight.  Guess what?  My original sensor also made my mixer work!  There was nothing wrong with my original Hall effect sensor except the connection between the control board and sensor was not adequate--unfortunately I didn't check the connections before I decided to buy another one. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. 
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall effect sensor looks like. 2.  You'll need a phillips head screwdriver to open up the mixer. 3.  Make sure that the connection between the control board and sensor is absolutely tight and snug. 4.  Also, make sure that the orientation of the sensor connection between the control board and sensor is correct. it's hard to know which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or facing down. If, after you've made the connections, the mixer still doesn't work.  Disconnect the sensor and reconnect it with the prongs facing the opposite direction and try it.  I actually found the right orientation through trial and error.  By going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work it were reconnected with correct orientation.   5.  This whole process probably takes less than 10-15 minutes to perform.

I had a similar problem where the mixer would also go to fast speed right away. On the back at the top was a screw that was loose. After tightening this screw I had 4 speeds but no stir speed. I unscrewed that screw and a bolt fell out. This bolt went to threads on the inside of the area covered by that back plate. I put that bolt back on and tightened it and the corresponding one on the other side and wala the mixer works perfectly at all speeds. Hope it helps. Make sure mixer is unplugged before attempting anything like this (wires are exposed when the back plate is removed - some kind of flexible circuit board).

I had the same issue and bought a new hall sensor / speed sensor. However, after replacing it, the problem wasn't resolved. I then noticed that the magnet on the motor shaft had moved out of position, and was not directly under the sensor. Once I moved this back in place, it worked with the old sensor... so now I have an extra sensor. I now need to glue/fix the magnet in place, as I would assume it will move out of place over time.

Ok to answer both of your questions. the mixer needs a new speed selector switch. it sounds to me like the contacts are burnt out or worn out one. the motor is fine its just the motor isnt getting the right voltage from the switch. its either on or off or medium speed or no speed. either way you both have to replace the switches. you can get the parts and do it yourself at kitchenaid.com or your local appliance store should be able to get them for you to.