DIAGNOSIS The potentiometer behind the control dial is failing or the dial's internal mechanism is broken. This is a physical wear issue.
FIX Unplug the blender. The dial pulls straight off. Check if the plastic shaft underneath is cracked. If the shaft is good, the issue is the potentiometer (variable resistor) on the control board. This requires disassembly. You need to remove the base plate and access the main PCB. The pot is soldered on.
IF THAT DIDN'T WORK The entire control board may need replacement. For a unit this old, it's often more cost-effective to send it to Vitamix for a flat-rate repair.
HEADS UP Do not spray cleaners directly into the dial area. Liquid ingress here will corrode the potentiometer and control board, causing permanent failure.