Well, your issues are certainly not typical. My first guess about the crashing and render errors would be memory issues. What are your system specs?
If you enable Defer Atmo, there's no point in increasing Detail, unless you want the *terrain* to look nicer; Detail has no effect on atmosphere when Defer Atmo/Clouds is enabled. I also would not recommend going straight to AA6 *and* enabling Defer Atmo. Try AA4 with Defer Atmo and atmosphere samples at 24 or 32. And turn Optimization back to Optimal (unless you turned it off for a specific reason; it will *not* improve noise to turn it off!).
- Oshyan