Apologies for double posting but if I add it to my last post nobody will see it.
Anyway I've been cutting down the campaign_script until I have a version that seems to remove errors in the system log and prevents battle CTDs.
(I can't totally eliminate them as different PCs will have different memory / GPU / monitor / graphics settings.)
The fatal graphics error has happened once but luckily I'd saved the game just before it happened so was able to re-fight the battle, this time without the graphics error happening.
I suspect it's to do with memory management.
I've played 100 turns with this campaign_script, and so far it seems reasonably robust, certainly much better than the beta version.
I had to remove the G5 really bad BAI, the named forts, assimilation script and one or two other smaller bits.
Turns are, of course much quicker as well.
campaign_script.txt
With so many factions that can be played there's always the possibility that problems may be specific to particular factions - I was playing Ifriqua.
(And no it's not much easier - I was always at war with my neighbors in every test run.
So give it a try and see how it goes for you.
Don't max out the graphics, I had mine on normal size units and reduced some of the graphics options to low.