Hoover wind tunnel

we have a wind tunnel "mach 4.7", I can't seem to find a model number. The unit turns off by itself, CONSTANTLY, after about 2 or 3 minutes of use. I keep it clean and clog free, but nothing seems to work. we even took it to a repairman who "fixed" it, which lasted about a month. The amount of dog hair it sucks up is incredible, but we run it on a pretty regular basis. We can only vacuum one room at a time because we need to let the damn thing "cool off". And we usually have to vacuum the rooms twice to get it all, so its takes all day to vacuum our house. any suggestions? Windtunnel Ultra U6425-900 vacuum shuts off after less than a minute of vacuuming; restarts after a cooling off period.  This only happens when using the floor brushes - not with attachments.  Recently had switch replaced, have replaced filters and bags with Hoover brand, all to no avail.  Any ideas?
Wandaq