Why would border select be selecting random vertices and edges. I just ran across this problem this morning. Could it be a add on that’s interfering? I tested Blender on a different computer and it doesn’t do it. Example would be if I add a plane and select the two right vertices it would select the bottom left with it. If I was to add a teapot and select a window of vertices at 6 o’ clock, it would select a random vertice at 3 o’clock.