The power head has developed an intermittent, but frequent, electrical short. Is there an easy way to assess where? Or is this a matter for a repair shop.
A majority of the time when this happens, it's caused by the hose having a short in it. You'll need to first identify what the source of the short it. To do this, turn the power brush on, and then swivel the hose on both the end before the handle, as well as the end that plugs into the vacuum, can you get the power brush to cut on/off for you from here? If yes, then you'll need to purchase a new hose. You may purchase this hose directly from Sears, however when you buy the hoses through Sears you have to assemble the hose yourself, which is very challenging. An easier way for you to replace the hose, is to purchase a Panasonic canister hose. There are two types of hoses, a 2 wire hose, and a 3 wire hose that fit most Kenmores. When you take the hose out of the canister, look on the part that plugs into the canister, how many prongs do you see here? If 2 then you'll need a 2 wire hose, if 3 then a 3 wire hose.
If you are not able to locate the short in the hose, then you may have a short either in the wand, or the power nozzle neck wire. For the wand, put one foot on your power nozzle while it's running, then pull up and down on the end of the wand that plugs into the canister, as well as the end that the hose plugs into. If you can get the vacuum power brush to cut on / off when you're doing this, this indicates your wand is bad and needs to be replaced. If it doesn't cut on / off, remove the wand and hose from the vacuum completely. Now look where the wand connects into the power nozzle. You should see two electrical prongs on the neck of the power nozzle. Do you see any burns, or black marks on the prong, or the plastic part that it comes out of? You can also test this part by turning the vacuum's neck to the left and to the right as if you were turning the vacuum.
Hopefully this helped you. If you need further help on figuring out which part is bad, please just use the clarification request here on Fixya, and I'll be happy to try and diagnose this further with you.