Pitching top A's - Aaaargh!!!

[trevor_john]

Hi! I've been playing cornet now for many years and am fairly accomplished. However one part of my playing has let me down so much that I now have a sort of Phobia towards it!
I have great difficulty pitching top A with any sort of pinpoint accuracy! All the notes in that area above and below present no difficulty whatsoever!
I use alternative fingering (3rd) on many occasions but that doesn't really solve the problem.
I have practised interval exercises so many times, and STILL it's a 'hit 'n miss' affair!
I understand that the pitching of top A has always been notoriously tricky but I also know that it can be mastered...but how?
Does anyone know of a tried and tested practice method of improving the pitching of these dreadful notes? Thanks