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Usually you only want one plugin trying to do the same thing at the same time, so two plugins for SEO can often cause problems and conflict with each other. I've heard good things about Yoast's plugin, but haven't used it myself. Similarly, you only want one caching plugin. My personal opinion is to try not to do too much all at once with this site. Get comfortable with creating a few pages before trying to add every fancy-looking plugin you can find. Have a regular contact page and ditch the inline contact widget and see if that reduces the number of errors. Remove the Quickbar. Remove the tag cloud that shows the stuffing of zip codes. Remove the RSS feeds of local news. Focus this site on computer repairs. Put up useful articles about how to do some basic maintenance on your computer, that type of thing.