This seems to be pretty common. Mine does it too. The problem is a small ball that, I think, acts as a check valve in the trans. As time goes on the ball wears down resulting in hard shifts from 1 to 2. The solution is to replace the ball. There's a faq in the archives about the cure. But I'm not sure if you've got to pull the trans to do it or if you can do it on the car. A trans flush will not help.