Missing Z Steps

My Makerfarm Prusa i3 8" seems to have a problem starting at about 10 mm in Z height missing steps. Both the Z steppers were blazing hot, so I got out the DMM and set the voltage on the wiper to 0.39 VDC per Colin's suggestion. I thought that the steppers were getting hot and shutting off because of the auto bed leveling. Tried a print last night and got somthing similar to the picture below. Display says it is at 44.2 mm height and my calipers say it is at 23.4 mm. Was printing from a Raspberry Pi Astroprint server, thought it might be that so I moved new GCODE to the SD card, unplugged the RPi and printed directly from it. Same problem.

20 mm calibration cubes come out fine. Other 40 mm prints come out fine. This is anywhere from a 5 to 12 hour print normally and I have 6 good ones, all within 1 mm at 60 mm heights. The failures are not consistant, I have re-sliced, diffierent slicers, everything I can think of.
http://3dprintboard.com/attachment.p...tid=5015&stc=1

Not sure what to check next.......

Makerfarm Prusa i3 8"
ZenmasterM auto bed leveling
Current Marlin firmware
E3d-v6 hotend 255° C
MK2b Dual Power (12v) heated bed 60° C
Taulman Bridge Nylon

Check your Z threaded rods for wear. I had a set get worn in one spot due to ABL, so that whenever the Z nut reached that area, it would slip.

I have had similar random skip issues till i added a head bed relay to unload some current from the RAMPS board. .

Unfortunately, it doesn't happen in the same spot and is not reproducible. I will try to get a picture of the other good prints and the fails to explain better. It seems to be more a 'motor heating and then skipping randomly' error. I did check the pots before I adjusted them and they were at .61 VDC. Can log term overpowering cause permanent damage to the steppers? So now when they get a little hot they skip? Also, I just remembered I have the tubing on the shafts, no zip ties or anything holding them.

That might be it!!!! As the motors heat up, the tubing slips? I'll try that. Is there a suitable coupler I could print? Most that I have seen are metal.http://3dprintboard.com/attachment.p...tid=5022&stc=1

Off topic, but I like your weather station "beehive". Did you design it, or get it online? I cut up some cheap plastic bowls to make mine.

It's my design. The current incarnation is some 0.79 bowls from Walmart. Working on a new sensor pack, so i figured i would build a new pagoda. Http://piwx.hamshadow.com

OP,

These were what was recommended to me. I have them in hand, my printer arrives hopefully Monday :)
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

Those are the couplers I have. Be warned, though, shipping will take a few weeks.

I ordered a pair. I'm going to replace the tubing for now and adding some zip ties until they get here.