These values don't make any sense. Are you sure you are using Atan2 method? Also keep in mind that this code makes use of radian angles (not degrees). Or maybe your rotationSpeed value is too high. In my project it was like 0.02f (i was calling this method every update).
#EDIT You can also try replacing that "else if" with "if" only. Or swapping Atan2 arguments from "src - target" to "target - src".