What are you washing when it happens - how many items, what type?
This sounds like an unbalanced load is being detected and causing the unit to prevent a high speed spin. This can happen on smaller loads or when washing single heavy items like towels, or bed sheets - it is a safety feature of all modern washing machines.
I find it will spin better with a fuller load - try adding a couple of towels into the load as well, this should help with the spin.
