In Mach3 under ports and pins on the Y axis change the check box for direction (if its an "x" change it to the checkmark or vise versa). This will cause it to drive in the opposite direction.
As for X jumping around and then faulting... If everything is wired up correctly and connections are tight then I would look into the pulse durations on the motor tuning and set them to a larger number until you get smooth motion. If its shutting down the drive then it could be the drive having some issue. Kind of hard to say without being there. You might check the debounce setting on the limit switches and increase it to see if you're just picking up noise.
I haven't used the spindle on function in Mach3 yet but it should be something to the effect of M3 S800. If it follows the format of the rest of the Gcode commands then it would be the code to turn it on and then the speed. However, I could be mistaken here. It could be that you don't have the correct port/pin assignment to get the M3 to actually turn anything on. I don't have a X3 (dang it!!) so I can't tell you how its wired.
"Good judgement comes from experience, and a lot of that comes from bad judgment." - Will Rogers