DIAGNOSIS The internal potentiometer (the component behind the dial that regulates speed) is likely worn out or has a dirty connection. Over thousands of twists, the electrical contacts can wear down or get dust/debris in them, causing intermittent power loss.
FIX First, try a thorough external clean. Unplug the blender. Take a can of compressed air and blast it into the small gap around the base of the speed dial. Then, slowly rotate the dial from 1 to 10 and back again about 20 times. This can sometimes scrape clean the internal contacts. Plug it back in and test.
IF THAT DIDN'T WORK If cleaning doesn't fix the loose feeling and random shut-offs, the control board or the potentiometer itself needs replacement. This is a repair for Vitamix customer service or a qualified small appliance technician. The 5200 is built to be serviced.
HEADS UP Don't try to pry off the speed dial knob yourself. It's not designed for user removal and you could damage the shaft or the internal components. If the problem is internal, contacting Vitamix support is your best bet—they have a good reputation for helping with older models.