My Hoover Windtunnel Ultra U6425-900

My Hoover Windtunnel Ultra U6425-900 began shutting off after a few minutes of vacuuming. The time interval became shorter and shorter. If I leave it sit a while, it will start again. This does not happen when I used the attachments - only with the floor brushes. The switch has recently been replaced and the whole vacuum serviced by a Hoover dealer about a year ago. I have been told by a local repair shop(not a Hoover dealer) that it is the motor, there is nothing to do other than replace the motor, and that this is a common problem with this vacuum. Hmmmm. Any idea if it really is the motor? I can take it back to the Hoover dealer to have it looked at, which I will if necessary, but it is a distance away. If it is the motor, and I can get a replacement, is it something I should attempt myself? I am a DIY'er and have had the vacuum apart before. Thanks for your help

Wanda