Pugliese

On my Arosa 1.4TDi I have a problem of changing down from 5th to 4th and from 4th to 3rd which is particularly bad. It seems as though it is not lined up. You try to change down say from 4th to 3rd and it will not let you, so go back into 4th and try again (engaging & disengaging the clutch again) and it will normally go into 3rd. Never any problems changing up, which is really smooth. No jumping out of gear or noise from box either. Car has done 140,000 miles so I guess the syncro could be on the way out. Was thinking of changing the oil and bleeding the clutch but has anyone else got any ideas? Thanks