What could cause a peak to split into 2 resonances as temperature increases in NMR spectroscopy I understand that some peaks may coalesce upon an increase in temperature due to an increase in free rotation, this does occur with some peaks in my data. Why might you see a peak split into two other peaks? I have thought it may be to do with an equilibrium shift in a rapid conversion between species but really I would expect that to show as one peak shrinking and another one growing. The observed effect is one peak clearly splitting into two. Any ideas why this could happen? 

Edit:
The sample is a diamagnetic complex with aromatic ligands with t-Bu groups in the meta and para positions. It is hard to assign peaks as I believe there may be an equilibrium between different conformations in solution. The peak that splits is in the alkyl region of the NMR (around 1.5 ppm) and probably corresponds to some part of the t-Bu groups. The 2 peaks produced do appear to get sharper with temperature and the two peaks are the same size, however the difference in chemical shift (between the 2 peaks) at higher temperature is much greater than the linewidth at the lower temperature (where it is one peak), so I'm not sure if this could be J-coupling being resolved.

If it was the case of 2 conformers being present, would you not expect the single peak to reduce in intensity as another separate peak appears and increases in temperature? Rather than a splitting into 2 new peaks?
Since I do not know which compound it is, my guess is that you have two different  $\ce{H^1}$ that have almost the same chemical shift at room temperature. The chemical shifts change with temperature as for instance in sucrose:



Here  are represented mainly the  $\ce{H^1}$  of the $\ce{-OH}$ but the $\ce{C-H}$ (the peak of 1) shifts also.  

In your case,  one is almost fixed, while the other one moves to higher $\delta$  and, therefore, they split into two. 

More info on the sucrose spectrum here.