Take a look at your servo monitor screen and make sure the switch is throwing full throw each way and there's no mixing touching it.

I would also check over my wiring for a loose connection.

Hell you could have caught a bad switch, bad components crop up from time to time. I've had a couple of them in regular service and they have been trouble free but I've seen brand new components be bad out of the box including some pricey servos and recievers so nothing is impossible.