Exactly what happened to me. Moved slowly sometimes or not at all till eventually they stopped. Easy fix once you get the motor out and the same motor (Lucas) was fitted to thousands of cars so replacement brushes are available.
viewtopic.php?f=27&t=9746&hilit=wipers&start=15
https://www.ebay.co.uk/itm/273059488207 ... R8T44JDYYQ