What is this 270k Ohm resistor? To drive 6 Amps through 270k would take 1,620,000 Volts and you would have to dissapate 9,720,000 Watts of heat. Obviously I am misunderstanding
I have much the same set up with a 2:1 belt reduction. I chose 180 oz-in Escap Disk Magnet motors because they hold their torque at speed. How fast are you trying to run it? Perhaps the motors are washing out?
Also don't uderstand 16500 steps per, is this steps/rev? I use 400 which seems suited to the accuracy of the machine. Perhaps you can't rattle the computer interface cable fast enough to keep up?