The liquid nitrogen seems to be able to get objects to go colder than itself. I dont think this will make much of a difference but i thought it might be worth pointing out.
I was playing about with Black Hole and Liquid Nitrogen and found that the Black Hole will go to absolute zero (-273). I think it also happens with other materials too.
uhmmm you brought a topic thats 2 months old back... but I guess its better than making a new one. Either way i've had similar problems with uranium, the pressure was 0.00 and it randomly heated up to 9000.