If you swapped the motors and the problem continued, I would highly doubt you have a "weak" motor(s).
Pushing the motors while energized does not damage the motor in any measurable way.
You haven't mentioned if you tested the motor cable, or the cable coming out of the motor. If there is a loose or intermittent connection anywhere along the motor cable, or in the pins on the connections themselves, it could produce the exact situation you are describing. Worse, it could actually damage the motor drive board that this particular motor is plugged into causing irreparable damage and you have to buy a lot more than a motor cable.
The only reason why all the motor drive cards could be "weak" would be a possible power supply problem in the drive box itself. If the drive cards are being under-voltaged this might cause the problem you are describing as well. I haven't seen it happen before, but you never know.
The last thing could be the computer itself. If the computer is lagging in the processing of step/direction commands to the signal generator, then it could be skipping steps, particularly during the deceleration/acceleration of the machine, causing the motor to hesitate momentarily (not moving at all) and then try and start instantly moving at full speed above 100 ipm. The motor can't physically accelerate 120 lbs from dead stop to 100+ ipm and would just bind. This would only happen, or happen more, when running a g-code program.
To test this, I would take another computer and install the drivers software onto it, then copy the setup file from the existing computer over to the new computer. If it works fine on the newer computer, the older one needs to be cleaned up or exchanged for a better one.
-Mike
Mike @ Torchmate.com | www.Torchmate.com
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