It may well be a damaged conductor in the power cable at the plug end - over time and with just slightly enthusiastic tugging the inner conductors become stretched at the bend point - they become thinner and thinner and eventually break - the ends still making intermittent contact.
Try cutting 6" or so from the plug end of the cable - if it's a moulded-on plug you will obviously need a new one.
This is a quick and easy thing to do and has resolved perhaps 75% of the vacuum claner faults I have looked at - and virtually no cost.
See...
http://www.howtomendit.com/answers.php?id=318957
Good luck...
Peccavi
June 2010