On my STI 5 getting into 5th isn't a problem during normal driving however, if you give it a bit of stick and try and get it into 5th quickly or even normally it crunches the box and takes a couple of attempts to get in any ideas?

 

The Phase 2 Box can suffer from the 5th gear retaining nut coming loose. This causes 5th to be difficult to engage and may also jumnp out of 5th on over-run.

Apparently you can get it sorted without taking the box out, just remove the rear cover off the gearbox.

 

...or the synchromesh is on its way out.

My My02 WRX went after 500 miles, without giving it loads of stick as it was being run in under 4000 rpm. The synchromesh was replaced and it's been excellent ever since.

 

THink this is fairly common, SG72 had the same problem on his Sti5 TypeR