What kind of computer and OS are you using?
You might try opening the task manager and clicking the performance tab to see how much CPU you are using. If it pegs, the motors will stall. Even reasonably powerful computers can have so many background tasks running that Machruns on leftovers.
Stop any antivirus/antispyware software while running Mach.
Make sure you do not have any printer drivers installed in the computer. That can also stall the motors when the driver polls the port.