Extruder Motor Twitching, not Printing

Does twitching mean moving very quickly back and forth just a degree or two?  Any weird sounds without filament?  Check the set screw to make sure it is tight?  Check to make sure it is plugged in all the way on the board?  Will it run fine if you plug it into a different motor driver.  Will other motors behave the same way if plugged into that driver?  Any videos?

easy to test it on another port, just unplug the extruder's plug from the main board and also the 'Z' plug below it then plug the extruder plug onto the 'Z' place on the main board. Make syre you unplug power to the printer first! both line power and usb power.

correction:

VIDEO
http://www.youtube.com/watch?v=G5-mk_e28dA

Plugged in normally...

Swapped the motors or motor drivers?  Have you checked the voltages on the board?  On the SD site there is a good description on checking those.

Sorry, still having a little bit of a hard time following everything you've tried perfectly.

It sounds like no matter where your stepper driver originally intended for the E (extruder motor) is located, it wont drive whatever motor is attached to it.  If this is true and the extruder motor works fine when attached to a different driver, any motor works fine when attached to a stepper driver that isn't the one originally intended for the extruder, and it doesn't matter where anything is located, then it sounds like you have a bad stepper driver.  They are easy to replace as I'm sure your'e aware since you've been swapping them around.  Like I said, not quite following exactly what you've done, but the whole point was to eliminate the main board as a cause, the voltage level as a cause, the motor as a cause, and then find out it if it was the stepper driver.  Since a failed stepper driver is not unheard of and it seems like you were able to plug other steppers in the same place on the board and they worked, I think you've found the problem.  Just like a car, you'd hate to start throwing money and effort into solving a problem if it isn't the cause.  I know Elmoret is pretty good at diagnosing the board related issues since he has worked with the schematics quite a bit, but my suggestion would be a new driver.  Maybe he will chime in before you order a new one.

for those of you following this...

the problem persists,
got the "new" board back, and the same problem,

but this time, when I swap the stepper motors, the swapped axis (the one with the extruder's chip)    does rotate,  just in a very jerky rhythm  (not a smooth spin like the other axis, but as though power is being pulsed to the engine instead of fed steadilly)

alas, no.

after the main board came back, the problem persisted, and I had to send the whole printer back to them for analysis...

haven't heard back from them yet.