The heat pipe appears to be doing it's job of moving the heat when things get hot. The problem is your fan controller is reacting much too slowly. It turned the fan off at the temperature it should of turned it on then waited until your GPU was cooking itself before turning the fan back on.

Also, in such a cold environment, 2875 RPM is much more cooling then you need and apparently more cooling than those heatpipes can take. Try slowing things down to just 1000 RPM or so and see if your pipes still freeze up. Ideally, you should be able to adjust RPM to keep everything just a couple degrees above freezing point.