Nerd Question

So, I'm working on modeling the dynamics of two trapped gases - fermions, condensed bosons, and thermal (above Tc) bosons.  I've finally worked most of the kinks out and now I want to increase the mesh size, but I am running into what seem to be memory/buffer limits- sometimes it seems to just 'time out', where the CPU usage has maxed out and then drops, and sometimes I get a segmentation fault... Anyway, some relevant details:

Fortran, complicated density profiles, iterating over loops of 250x250x250x10, run on my PowerPC Powerbook running Tiger.