Are you using the xml you attached or is it a different one? If you are then that will cause problems in as much as accuracy is concerned and that should be your first step to resolve any issues.
As for the 5v, that should be fine as the requirements of the power supply should just be milli amps, however I should qualify that by saying I am not familiar with the CNC4PC boards and their requirements. It is more likely to be your motors power supply that is an issue if you are not able to move two or more axis at one time but all are fine when moved individually. Put a voltmeter on the drives power input terminals and jog one axis and see what it reads, then jog two axis and see if the voltage drops.
Another thing that could be causing issues is if your Step/Dir cables are not shielded and properly grounded at one single point. This could cause noise to be introduced when you are moving multiple axis.
Hood