A wise old dog man once posted " if you have a good dog that has been performing consistently for that long, you better consider his health if he suddenly starts performing poorly". More than likely it's a health issue if he has been performing well for that long.
Heart worms and brucilosis come to mind on this one. I could be off on this but he sounds like he is worth getting checked out by a vet.
Keep us posted on it.
X2