Hello @Mark C
This could be caused by a couple of reasons.
In order for us to get you the correct information we would need a bit more information. Please confirm is this issue occurs with all cleaner heads or just the motor bar cleaner head.
I think you will find the link below would also be helpful. Even though your brush bar is spinning to fast, the steps are the same as if it spins intermittently.
YouTube
The Dyson YouTube channel hosts a vast amount of content that can support on this process. As well as those that provide tips on using, maintaining, and getting the best out of your Dyson machine. Simply use the on-screen search option or navigate through the playlists to start viewing these.
If you still need help, feel free to post a question to our wider community. The best place to do this is in our Ask the Community forum.