KM005 Titanium Major - Motor jumps during usage, leading to loud noise

Hi

I have disassembled my mixer to try and work out why the gears jump every use, leading to a loud noise. The pulley is movable and I am trying to find out if that is meant to be so, or if it meant to be tighter. If the pulley (with pin) has gone, I will certainly need a new one, but I am looking for some advice as to how to diagnose this correctly. Is this something you can help with?

Thanks in advance