I think the thing is with your dynamic repeater creation. You have to make shure you recreate it in Page Load everytime the page postbacks. 'Till this event only controls that are in aspx are put into page control tree. Everytime that server sends data to client the whole page is disposed, so you have to recreate everything again.

You must create Item template by adding other controls to it. i.e.
repeater_object.ItemTemplate.Controls.Add(some_control)...
but don't forget to give them ID's.

In ItemDataBound event you can still do the FindControl... If the server gives other actual ID's try displaying all ID's of the item to see if your item resides inside ItemTemplate...

FindControl iteself won't raise exception, but casting and using refference may if FindControl returns null. Probably this is your problem.