I have a 5-year-old DC 21 which we have been very happy with. Unfortunately over the last month or so the power head cuts out after few minutes of use. Pushing the switch on the power head it turns back on, but again only for a few minutes before it cuts out again. I have stripped down the power head to make sure there is nothing binding it. I then resorted to buying a new power head, but unfortunately the new one does exactly the same thing! So I'm guessing there is a problem in the power delivery, either in the main unit or in the hose wiring somewhere. Has anyone had experience with this problem? Any suggestions for where to look first and how to track this down? Thanks in advance.