One place I've taken my car to says its to do with how the clutch was installed. It is possible when trying to refit the gearbox to hang the gearbox shaft off the clutch plate while trying to line it up. This can bend the fingers on the clutch plate a small amount which will cause the clutch to not disengage properly.
Eitehr adjust the clutch to lessen the problem or you might have to replace it.
Take the car to a good clutch/brake place thats involved in performance stuff like motorsport & let them drive it, they'll soon let you know.