You're ground looping. Let me guess, a small swerve to the left gets deeper, you increase the right rudder and the plane adbruptly swerves to the right, then back to the left, then to the right and so on with each swerve getting a little deeper [X(].

Most of the advice you've been given in the previous posts is on so I won't belabor the point, but I'll add a little advice of my own. If you start to loose it and get into this swerving dance, then chop the throttle. Turn around, taxi back and try again. You have to time when you put the rudder in and when you let off so you don't get out of phase with the plane and make things worse. It's hard to explain, you can only learn by practice.

I used to have this problem with my Decathalon. Now I don't. I can't tell you exactly what I'm doing differently, but I've taken it off 100's of times. It must just be the practice.