cms:CMSRepeater already has a pager built-in. I suggest using a basic repeater to attach your datasource and pager to. This has worked much better for me in the past.
If you wish to use the repeater you have, go with a minimalist approach on setting properties. Remove the following:
RepItems.ControlContext = ControlContext;
RepItems.EnablePaging = true; // tells the repeater to use the built-in pager
RepItems.PageSize = PagingPageSize; // setting the built-in pager properties
RepItems.UniPagerControl = Pager;
Don't set any of the pager properties on the repeater as those will assign/set values for the built-in pager of the repeater.