Hmm, I'd have this function setup a bit more nested, but that's about it - and that's not the problem
Have you tried tracing the _width at the end of the function? You might have to wait a little before all the traces are done (after you close) but those should give you a better understanding. Something else to consider is using scale in stead of width - don't know the exact syntax anymore, I'm used to AS3 by now. Try something like scaleX = -scaleX (which is AS3, but AS2 should be something like it)