You're probably losing steps on the rapids. With some controllers, the speed of rapid (G00) moves is set separately from that of ordinary (G01) moves. Acceleration also is a factor; if your motors seem jerky, it might be set too high. Those motors, while high in torque if they get enough voltage, can also be problematic if they don't.
If you're using these motors: http://www.longs-motor.com/productin...12_86_163.aspx you'll notice that they have 22 mH inductance. That means that to run at optimal speed without overheating, they want about 150v DC supplied to them. How much are they getting? For plasma cutting, which doesn't involve a lot of counter-force, you'd probably be better off with motors with less holding torque and less (a lot less) inductance.