It's going to sound strange (I thought it was, too, when I first read about it) but I fixed my Clear Sky crashes (with and without mods, including Clear Sky Complete) by uninstalling a very specific Microsoft Update, namely "Update for Microsoft Windows KB2670838".
I was having crashes in Clear Sky left and right, even though my system isn't over-clocked (maybe yours is, however, is it? If it is maybe it's related to an over-clock instability, who knows) and none of my games crash (haven't had a game crashing on me in months). So I looked around with Google, searched some forums and eventually stumbled upon that, the "KB2670838" update is supposedly known to cause crashing with Clear Sky.
I found the update in question in Control Panel > Programs > Programs and Features > Installed Updates. From that list I scrolled down and found the update in the list and uninstalled it. I then rebooted the computer, it did some changes and then got back to the desktop normally (it's just one update you manually remove anyway, nothing important). I have been able to play Clear Sky without crashes since then.
Note that the update in question is not a "Security" update, it's just a normal update, in the list the exact name should be "Update for Microsoft Windows (KB2670838)", not the "Security" ones.
So anyway, maybe in your case the problem is not at all related to that update, I'm not sure but at least maybe you can try that if you think you've tried everything else so far. If not, try to play the game in DX9 mode first (maybe just doing that will do the trick), or maybe you can try to temporarily turn all the NVIDIA (or AMD if you got a Radeon card) control panel's 3D settings to default (if you made any changes to the "Global" settings, etc).
Hope it helps.
