+1 on the wires... Its a somewhat known problem that these can be subject to loose connections, short circuits, noise, etc. which can cause temp errors and or/weird temp readings...
Check the ones connecting your temp sensor with the small amplifier board on top of your print head, as well as the ones that connect that board with your um main board underneath the printer.
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peetersm 124
I had a similar issue and it turned out to be the wires. I switched to the spare set of wires and I'm okay now.
See what GR5 told me in this thread for better details.
http://umforum.ultimaker.com/index.php?/topic/5617-um1-mintemp-fail-at-random-times-solved/
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