You should be able to establish whether the disk is loose by checking the assembly of the screw and Belleville washer. As sparker said, be sure the washer is on the right way up. If it has been running loose, the thick washer may be slightly eaten away and have to be replaced. If the disk is properly attached, check for a sheared flywheel key causing advanced ignition timing. The two pieces of a sheared key can lock together with the flywheel something like 10 degrees clockwise of where it should be - the amount of advance is determined by the thickness of the key.
