Replace the potentiometer with a better one. Do not use the CHEAP PIECE OF [email protected] that comes with the blender. For what they charge for these machines, they should be using MUCH better components. I used part RV4N502C-ND from Digi-key as described by another poster for a different model. The knob didn't fit, so I also used Mouser part #5164-1500 knob, which look exactly the same.
It will probably just need the pot replacing, take note of where the wires go. I see someone else here is doing the same job and wants to know where to put them back. Take a photo first saves a lot of grief. The pot is the rotary resistance switch behind the knob.
I would suspect a motor fault, most likely an insulation problem in the armature or the fields. Contact Vitamix for your local authorised service agent. Unless it has been assembled incorrectly it should not be one of the internal wires as they are supposed to be secured and correctly routed during assembly.
Stiff or stuck blade problems can be fixed by replacing the two bearings and seal inside the blade assembly. Care must be taken in installing the new bearings as not to damage them. Ideally they should be pressed in, on a press, but they can carefully be tapped in using a sockets to match inner and outer races so your not tapping on the balls.