Try using
<cms:CMSRepeaterWithEffect ID="CMSRepeater2" runat="server" ClassNames="cms.news" TransformationName="cms.news.preview" EnablePaging="true"
SelectedItemTransformationName="cms.news.default" ItemSeparator="<hr/>" Pagesize="20">
</cms:CMSRepeater>
There are a few differences between the two web parts, but I find paging to just work a lot better on the Repeater With effect. I'm not 100% convinced that the standard CMSRepeater web part even does anything with those properties without being hooked up to a separate paging control in some way, I could be wrong about that last part though.