I'm certainly not an expert on this, but the few times I had issues with the servo, it was either incorrectly wired (which happened far more than I care to admit), or the wire connections were loose, which kept it going back and forth. But sounds like the board is receiving an errand signal?
I did just upload the program on a new Arduino, and the program seems to be working. May be worth trying to reupload, double checking the wire config, and saying if any loose connections. I ended up getting a solderable breadboard because I couldn't keep the connections secured enough (probably bought cheap breadboards).