Thank you all for your suggestions.  My machine has 16G of memory so I was real surprised at the suggestion that I am running out of memory.  The system monitor is indicating that there is still a lot of unused memory available.  However, after reading some of the comments, I realize I am using high resolultion texture maps in my terrain shaders (about 1M each) and the render settings are cranked up (see file).  I will try dialing down some of the render settings to see if that works.  If I turn GI totally off, it works OK, but does not provide the lighting I am looking for.

If you further suggestions, please let me know.