My print always stops-cold about 4cm up on the Z-axis

...even when they are going just fine!

-I though of overheating but I have a fan, heatsinks and thermal paste on the polulus.
-I tried a hollow model to see if time/workload had anything to do with it but no love. I have also done demanding horizontal prints just fine.
-The fan on the powersupply keeps whirring, but Pronterface says "disconnected", and when I try to steer manually it locks up..
-I can manually steer it up to more than twice that height
-the "build dimensions" in settings are 200x200x200, but I bug out at about 4

Any hints? i thought maybe a data... thing? memory something something?

(Mendel running RAMPS 1.4 and pronterface)

Thankyou all!

If you have the option to print from a SD card on your machine (I don't know much about the individual electronics, only about mine which is Melzi) you should try to print from it... I had communication issues early on and these were all using a laptop to communicate over USB, I quickly learned to copy to the SD card whenever i had a large program or was leaving the printer un-attended... but you may have a worse scenario where it is constantly failing...

Worth a try, may not be it, but if it helps you know you have a communication issue.

It is definitely disconnecting from your computer, which either shows a communication issue or that the printer itself has shut down / rebooted - if the power is still on to the printer (fan is running, hot end / heated bed still heating) then my guess is communication, if all the things I mentioned have also stopped (machine is cooling down etc) I'd say it's reset itself...

Try this out, see how you go - and someone with more knowledge than myself will no doubt weigh in on this very soon ;-)

I remember reading something about windows can time out the usb port....

just a thought, if you use skeinforge it has a maximum layer height limit which needs to be set very large so that it is not reached by the objects to be carved, but keeps it from running for days on a model scaled too big etc.
skeinforge: carve, layers to
That is my best guess?

Or another idea:

how thick are your extruder cables to the ramps?
Maybe its requiring too much amperage and your cables may be too thin to withstand a prolonged extrusion (resistance heating)

Just some thoughts because I had a similar problem with the heating bed before and the extruder as well...

try using thicker cables. I use 18 gauge stranded, but you may use a bit thinner if it works for you. I just wanted it to warm up as fast as possible.

cheers and good luck