I bought PC disc in April this year. It grinds and clatters just as described. It stubbornly refuses to change gear down especially when going up hills when lower gear is most needed. 50 cycles suggested I lubricate gear change cable. This had no effect.
After reading these I turned bike upside down and cleared all sprockets and chain with engine degreaser. Then lubricated everything with silicon spray grease. This helped a lot but it is still a bit noisy and gear change is still hesitant.
As stated by others it runs perfectly with power off. Course I did not spend two grand on electric bike to cyle without power! Rather disappointing.