SOLVED!!!!
Ok, I took the sprite advice by AngryTitusPullo and also dhuros' comments that the game works fine for him (I am suspecting he doesn't have the Promethean Mod) and examined each and every sprite entry....and found a missing one! The roman medium cav sprite is referenced, by not included in my download. Damn. I forgot to include it when I integrated the Promethean Mod.
So, I took HopliteElite's save game, and tried this test twice:
1. Attack with his Greek stack.
2. Attack with his N. Italy stack.
3. End turn.
4. Attack with his Greek stack.
CRASH
Then, I changed the sprite for the medium cav to be the equites sprites, and repeated the above test twice...no crash.
So, I'm sure that's it.
So, if you guys want to test this, open your descr_model_battle.txt file, and rename the roman_medium_cavalry sprite from
romans_julii, 60.0, data/sprites/RG/romans_julii_roman_medium_cavalry_sprite.spr
to this:
romans_julii, 60.0, data/sprites/RG/romans_julii_roman_equites_sprite.spr
And then test your saved-game. It should work. I've tested this with six battles in a row, no crashes now. I'm uploading a new hotfix now.
EDIT: HOTFIX HAS BEEN UPLOADED. See my main thread for the link and instructions.