jumping out of 6th gear

i have an issue with 6th gear. when driving through a town the other day, i shifted down from 6th to 4th to reduce speed, when out of the town and back up to 110kph the car started to jump out of 6th gear. This happened 3 times in a row, on the 3rd time I kept my hand on the gear level and could feel in thumbing until it jumped out. Only happened in 6th all the gears are fine. Car is a 2008 gt liberty sti.
Has anyone else experience this or any ideas on the problem? My regular mechanic doesn't do gear boxes, so I'm looking for a subaru gearbox specialist in sydney - any suggestions would be helpful (googling sti transmission doesn't return what i'm looking for)
Has anyone else experience this or any ideas on the problem? My regular mechanic doesn't do gear boxes, so I'm looking for a subaru gearbox specialist in sydney - any suggestions would be helpful (googling sti transmission doesn't return what i'm looking for)