I have the exact same problem on my left hamstring. I have a Fizik Saddle, which I love to death b/c I got it from sponsorship but even since the saddle change. I get the exact feeling in my hamspring like you do. Its in the top by the butt. I am going to try lowering my saddle to see if that works. Also I use to have a aspide arrowhead which I loved to death but it just didn't cut it when you rode more then 60 miles. The new arione is cool. I just got it and it is great, except for the hamspring problem.