Sorry I forgot to add the error message I am getting. It is:
The Iterative Solver stopped.
Numerical difficulties.(Negative diagonal term)
Possible large variation in thermal material properties.
No results saved.
In addition, the correct range on the cooling curve is 410W @ 300K to 4W @ 12K.
so if i understand you correctly:
1. the results for the shorter duration don't match your expectations
2. you can't get it to run the full length that you want
i'd focus on #1 first. how do you do this? bring the problem down to a more simple problem to start. get rid of the temp dependent materials, get rid of transient, get rid of the heat input curves. does a basic problem solve?
then you can start adding complexity. sounds like to me that you've got an input problem because i've run similar studies with those conditions with success before. but potentially your combination is affected by a software issue.
something to consider. rather than waiting a long time for something to have an issue. you can consider breaking your thermal analysis until multiple studies. if there is a limit on your study, you could do 6hrs in one and 2hrs in the other.