The page loading an ASCX file via an object created of type Control becomes complicated when you need to programmatically manipulate this ASCX file as almost certainly this object of type control will have to be cast into the type of the ASCX file. The complications are centered upon the temptation to cache this control.

This aspnetpro.com article,"Dynamically Load Cached User Controls (and Live to Tell About It)," addresses this problem here:

http://www.aspnetpro.com/NewsletterArticle/
    2003/08/asp200308pj_l/asp200308pj_l.asp