Vibrating stepper motors with Farmduino 1.5

Since last autumn I have a strange problem mostly with the Z axis: The stepper motor only vibrates but there is no smooth movement. When this problem occurs on the Z axis it always moves downwards because of its own weight, even if the command was to go up. Rarely I observe the same problem with the Y axis where no movement occurs (Y is not affected by gravity). The strange thing is that the problem does not always persist. Sometimes my Z axis works just fine. Y is working most of the time. I have never seen the same problem with the X axis.

I uploaded a video to YouTube which shows the failing Z axis and it also shows one of the moments when its working: FarmBot error: Z axis vibrating but not turning - YouTube

I did some tests to isolate the cause of the problem: I tested the stepper motor separately and it works just fine. I also tested the cable and it works as expected. In the settings I deactivated the encoder but that does not solve the problem (the encoder correctly detects a problem with the movement). I also deactivated quiet mode which had no effect on this problem. I played around with different motor currents and with 1550 milliamps I had the best performance. Below 1400 the force was not sufficient to move the axis upwards, above 1600 the current value seems to lead to less force instead of more (maybe there is an issue with the way the motor driver interprets high values sent by the firmware).

Sometimes the Z axis stopped to move at all when I set the Z motor to be always active. I suspect this complete shutdown was related to a motor driver getting too hot. I added small heatsinks which seem to have solved the issue. However these complete shutdowns of the driver seem to be unrelated to the vibration problem. It took me some time to come to the conclusion that these problems are probably not related.

Does anybody have any idea where the vibration problem might come from? Am I the only one having this issue (maybe my Farmduino 1.5 has a strange electrical problem)?

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