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The ability to call a random article is already built into the txp:article_custom tag, with the attribute
<txp:article_custom form="form_of_your_choice" limit="4" sortby="rand()" sortdir="desc" />
You can also call this attribute from the txp:chh_article_custom plugin.
It is now also possible to call random articles with Rob Sable’s rss_suparchive.
Archived [?]: functionality now in core. "sort" replaces sortby and sortdir