To change baud rate, I'm afraid you'll need to recompile Marlin. I'd upgrade to newest version while doing that. I've had some problems with longer USB cables and 250000 br, but with short cables (like the ones that come with ramps, ~20cm or so) I've been running that speed happily. Of course the short cables aren't any use if there's no PC nearby. I use Raspi and OctoPrint so that's not a problem.
Oh and make sure there's no power cables near the USB cable, I've really interesting problems when that happens.