Check the gearbox has enough oil in it.
Does it happen when the engines/gearbox has bene going for some time ie the oils warm?
Did you use the right or semi right gear oil spec wise?
Aircraft have sligtly different requirments.its like it weather its been runnin 2 minutes or 2 hours even wen the car engine is turnned off u cant push it into 5th...i might have used the wrong oil i used stuff from work that we use in gear boxes for air crafts engines blokes at work said it would be alright.. wat oil should i use coz it might just be the fact i didnt out enough in or its the wrong stuff!