My Impreza drives fine at low revs, gearchanges are smooth, but over 5000 revs, gearchanges crunch going into 4th and 5th?

Its had a new clutch but the problem was there before and after so it cant be that, the gearbox oil has been changed too.

Anyone have any other ideas?

 

WHAT WAS PUT IN? If it was full-synthetic, drain it out and re-fill with a good semi-synthetic such as Castrol Driveline. It's known that the synchromesh performance is compromised in many boxes by the use of a full-syn gear oil. (my previous 99MY suffered this)

jOHNd

 

Common problem 5th gear locking nut needs doing up - can be done with main box in car by someone who knows what they are doing