just took a look at your capx, and when the Roc is flying to the left, it is never hitting the centerline, or the farleftPt. The path ends before it makes contact with the line, so it never sets a new path. Probably because of where origins are... or something like that. Also, the Roc doesn't seem to want to fly very close to the left edge of the screen.
So, I moved the farleftPt out from the edge (set its x to 25). And when finding a path to the left, I told it to subtract 50 from the x of the target to make sure it was going to hit the line: Find Path to (centerLine.X -50, centerLine.Y) or Find Path to (FarLeftPt.X -50, FarLeftPt.Y)
I didn't have to change the path finding when it was flying to the right.
After those changes it continuously flew back and forth just fine.