My 620i.e. just started giving me problems with 4th gear. All the other gears seem to shift smoothly, but 4th does not want to drop in without alittle " grinding" , I've tried it at different speeds & RPM's and it still seems to not want to go smoothly. But why only 4th.? The master cylinder level is at the top of the sight window. The slave cylinder is not leaking.Could it be " operator error"? I really don't think so, never had a problem before. It is 600 miles away from its 6000 mile service. Should I take it in early???