Everything you describe re: intermittent working/not working still (IMO) points at electrical problems. There is nothing in Mach that would cause a motor to just stop for no reason. When your motor just stopped did the DRO still show movement? I'd be very surprised if it didn't.
However, there are some inconsistencies in your xml which may explain some of the things you've described specifically re: homing.
You have A slaved to X with "home slave with master axis" un-checked. This means you're configured for "gantry squaring". However you don't have any home/limits configured for the A axis. I don't use homing gantry squaring but I think I'm right in saying this is not going to end well. When X hits the home switch, A will just keep on going and try to twist your machine apart. Maybe someone will confirm/correct that.
My best advice is double and triple check all your wiring and then sit down and read the manuals and watch the videos so that you can understand things like this in your config. if you haven't seen them yet they're here
http://www.machsupport.com/help-learning. By all means come back if there's anything you don't understand but give them a shot first - it'll really help you to understand how things are supposed to work.