I have the same problem. I can climb like a goat and also accelerate very quickly, but have a hard time keeping up on the flats. I think for me it could be a combination of me and my 650 wheels - not sure. I have short, bulky legs which I think are more designed for intense, short spurt types of things, but I am training for a century this year and would prefer to have a powerful endurance. I hate not being able to keep up, too!