there is your problem. i'm afraid you're going to need a lot of light and/or high iso to get a decent shutter speed with such a lens, especially at the telephoto end.
a consumer zoom with such a long range is always going to be an optical compromise and so will be little soft too.
your 400d doesn't have great high iso performance so you may want to consider hiring a better lens which lets in more light the next time you want to do a shoot so you can get the shutter speeds down and freeze the action.
i'd give these consideration (they are similar in range to what you have):
Canon EF 70-200mm f/4.0 L
Canon EF 70-200mm f/2.8 L (you can also use this with the 1.4 extender, to get more reach if you need it and it will still be a f/4 fast lens)
these are both fantastic lenses, you'll hear many people on this forum singing their praises and if you get on well with them, you can save up and get one. the F4 version is actually quite inexpensive for what you get.