The CPU, stepper motor drivers, and logic and motor power supplies are all on the same circuit board so the fault won't be with the wiring. The drive current is controlled by four sets of three resistors in parallel for both pan and tilt. If there is a fault with any of these resistors then the driver chip will supply less current to the motor resulting in a loss of torque. To test your resistors you'll need to take the driver chip out of its socket and measure the resistance between ground and four other pins. There are four ground pins you can use (pins 5, 6, 17, and 18), and you'll need to measure the resistance from any one of these to pins 9, 10, 13, and 14. You should get about 0.5 Ohms. Anything higher than that means you have a problem. This method only works for the pan and tilt circuits as they use the PBL3772N driver chip. All other stepper motors use the PBL3775 chip which has the same ground pins, but you only need to check the resistance to pins 2 and 21, which should be 2.2 Ohms. Its well worth signing up to lightspares.com as they have part and circuit diagrams for all sorts of fixtures. They have the circuit diagram for both versions of Mac600s, for example, which will help you with troubleshooting your lights.