I noticed while playing a large map game from turn 250-300 that the frequency of my out of memory errors was directly related to how often I saved.  I have auto-save set to every 3 turns and it seemed that the out of memory errors always occurred on an auto-save turn or when I tried to quick-save the game.  The save was never successful either so I had to go back to the previous save.

Loading also seemed to cause problems and speed up out of memory errors.  For example, loading a game and casting Curgen's Inferno then reloading the same save results in the fire-storm/lava animation remaining at the location where Curgen's Inferno was cast prior to loading.

I don't know if this helps, or is entirely correct, but for me there is a strong correlation between the frequency of saving and crashing from out of memory errors.

side note: this produces an interesting dilemma, save often so as to not lose too many turns when it crashes, or save less often so it doesn't crash as much.