Honda HRB475 self drive slipping?

My Honda HRB475 slips intermittently when I engage the self drive, and gets slightly worse as the grass box gets fuller (i.e it puls and stops, pulls and stops in quick succession). The belt is fairly new and the cable is too, so I guess it may be the central belt pulley above the blade that may need replacing, as it looks very shiny inside and worn slightly (the pulley at the drive cable end is new).
Has anyone had the same problem, and could I be right?
Thanks