Quite often I receive a warning symbol (red exclamation point) on top of the 'lock' when I access certain sites. When I mouse over the symbol, I receive a message "Warning: Contains unauthenticated content". Is there a fix for this problem?

Yes it is alarming. A "supposedly" secure web page that includes content from non-secure servers. Complain to the webmaster of those websites. If you right-click those pages and select View Page Source, you can use "Find" to locate the addresses of the servers delivering non-secure data for that page. Use '''http''' as the filter in the Find bar to locate those "http" addresses - you'll also see all the "secure" addresses - '''https''', as you use "Next" to advance down the source of the page.