On the vacs I have looked at over the years the most common fault is a wire breaking inside the cable - caused by repeated tugging - sometimes at the plug end and sometimes at the machine end - the fix is to work out which end and then shorten the cable by 6" or so. Wriggling the cable usually diagnoses which end.
You may indeed have a dodgy switch but my money would be on the cable itself.
Dysonmedic.com has words and pictures showing how to change the switch...
Good luck...
Peccavi, September 2010
