I had this problem when i went to the drags the other day. It ended up being a cracked pressureplate.
Get under when its doing it and see if the clutch slave is getting jambed or not.
if not, theres a good chance its inside the box.
I thought my problem was either the master or slave originally so i went down to the clutch place. They said the main reason for this is that the master cylinder doesnt let all the pressure out of the system, but you say that have been replace so it shouldnt be that.
