Kinda makes me wonder about the pressure plate or flywheel as well. If it barely vibrates when it's at one spot, but worse at another, my mind tells me there may be an imbalance somewhere else as well, that either compounds the vibration or eliminates it. My thoughts go with the disc itself or something inside the trans possibly being out of balance, then the pressure plate or flywheel being out of balance. You may be able to have the pressure plate and flywheel checked for balance if everything looks to be intact.