I was just reading an old post along the lines of the suction release valve was constantly engaged resulting in poor suction. It was found to be a blockage in the machine, which prevents the motor overheating.
But I just wanted to add I have come across a couple of faulty springs before now, and as soon as the dc07 is switched on you can hear the valve releasing and loosing suction. I've just popped on a different filter holder and it's fine.
Hope that helps anyone as this could be a last resort for you.
DC01, DC02, DC03, DC04, DC05, DC07 + USA DC07, DC11, DC15, DC16, DC17, DC18, DC24, DC25, DC26, DC27, DC41, CYCLON !!