Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Always be quick in dealing with levels! At first take your time to work out how to drop the pieces to complete it. Then when you know what you are doing go back and start the level over and do it as fast as possible.
Use the small reset button on the bottom right of the screen, this resets the clock and lets you start over.
Play the game, practice and you will soon be completing all the levels.