If you're scrambling at the end of an iteration to complete quality control tasks in order to accept user stories then you need to shift those left. Split the user stories into smaller chunks (as long as they still deliver value), have multiple team members work together on a single story to complete all the tasks (swarming), and write the automated test scripts in parallel with coding or even before (TDD). People sometimes try to tell me that this approach is impractical or won't work, and yet when I led Scrum teams and forced them to do it that way it produced good results and the team members were happier.