ls1's do this ALL the time.....
almost always its the pressure plate thats collapsed on one side, so when you push the clutch, it only depresses half the fingers. allowing for most of the drive to discontinue, but not all.
thats why, if you hold the brakes on, it will let go, cause the clutch is slipping enough to stop the drive. definetly not a good idea.
i reakon your going to have to remove the box. after you do so, just look at the fingers and see if some are lower or higher than others. this will give a clear sign if its collapsed or not.
another classic ls1 problem is the pressure plate bolts breaking and not providing even pressure on the clutch plate.
(sorry for all the ls1 reference, but it sounds like exactly the same thing, and thats what i know.....ls1's, all too well)
