Thanks for the link...
It doesnt seem to be on a hill or anything, and it doesnt drive past the wire, it seems to stop slightly past the wire with the "out of border" error. I ve been watchin him a little more since i posted, and i think it might simply be the grooves he leaves in the grass.
When he is driving towards the border he has to cross over his perimeter tracks. When he approaches them he kinda bounces a bit since the grass is bumpy from him constantly driving the perimeter. I think these bounces throw him off and every now and again he goes just that little bit too far past the wire.
I have not had an error since my last post, but i think this fall i might have to top dress the perimter of the yard to fix his "grooves".