When my CNC router is moving with the G Code it is jerking with each line!!!! The Constant Velocity button is turned on. I have to slow it down and run the machine about 1/4 of its full capacity. I honestly don't know what else to do. The machine works fine when using 3 axis and we can run it at full speed but as soon as we introduce the A & B axis then we are having to slow it down.... its like mach 3 cant supply enough data fast enough to the machine...
Help i need to get this thing going properly!!!

Try this action, may help...
Diselect the option Config->Toolpath->Tool Position

I have seen jerking movement because of this in the past.
John