Just a couple guesses here but I'm thinking possible tight pilot bushing ? I noticed the pilot bushing I removed was too long. I replaced it with the proper length. I clutch tool slide in without binding. If the driven disk or the pressure plate are not perfectly flat then that could cause intermittent clutch grabbing. With 2 different drive discs and pressure plates (new and old) there was chatter. Also bell house mis-alignment and flywheel surface could cause intermittent grabbing. I agree, I wish I had sat the B-housing on a flat surface before installing. Clutch pressure plate not adjusted ? 1" of pedal free play. I may give the pedal more free play and see what happens. Is this diaphram type or 3 finger ? The one I removed was a 3 finger. The new one I installed is a diaphragm type.