When/how are the crashes happening? After you build a certain thing? When you fight? When you build a city?
Also, are you reloading the game after you switch to Hyborem? Switching Civilizations causes some small glitches, primarily it resets all game options internally, but doesn't reflect this fact in the options screen. If you log out and back in then it clears it up though. It might be that your computer doesn't deal well with the default settings.
To be honest with you, I'm not even sure because it crashes while in the process of going through the turn. In the custom game one, I simply did nothing because I was trying to make it a controlled situation and I crashed anyways.
My comp is pretty good and if you mean default graphics settings, I have them on max essentially.
As for the first save, I was basically biding my time, so I wasn't fighting and AFAIK nothing got built or completed researching at that time of the crash. I also have the sound off, so maybe I should have it on because I've read others having some strange sound (or otherwise would be unusual) right before the crash, so that could be a clue.
It's just that it's frustrating to not know what is going on or how exactly to pin it down as there are so many factors.
Also, I'm not reloading the game or restarting my comp after switching to hyborem, although I did do a savegame, but that doesn't stop it from crashing.
I tried a game where I did get hyborem out but didn't switch and did not crash so.... no idea.
edit: I didn't get any odd sound with one of the saves, but I'll try another.
edit: hmm, I wonder if doing the exit game and reload because I tried that and the wait at end of turn didn't change (I didn't change it before the save), so maybe there's something going on there? have yet to see if a crash happens as I just reloaded it.
EDIT: crashed two turns later

I wonder if it has something to do with the spread of hell terrain.