As I began in part 1, seemingly “sporadic” query problems are often just straightforward failures being masked by the three levels of load balancing involved with a SharePoint 2010 Search Query and my goal here has been to help unravel all the moving pieces. Here, I described the load balancing that occurs when there are multiple instances of the SQ&SS (e.g. Farm level load balancing) and from this, begin to explain why the "Internal Server Error Exception Occurred" failure is completely ambiguous.
There is an undocumented assumption baked into SharePoint Search that the Default Public URL of a Web Application will be crawled. If you want everything to work auto-magically, crawl the Web Application's Public URL for the Default zone. Otherwise, when crawling a non-Default zone, expect things to break. Read here to understand why.