This sounds like the adaptor, when tightened, is contacting the base plate under the deck. The Toros have a spacer inside the blade hub for this reason. Did you switch blade hubs? The only time I've ever seen a blade consistantly come loose is when a mower hits a brick, sprinkler head or something else solid. This shears the soft aluminum flywheel key. Then after installing a new blade, then attempting to start, and because the sheared flywheel key offsets the timing, the engine fires backwards which is oppisite the blade/hub/bolt threads, and essentually unscrews the blade bolt with a shock like that of an impact gun. This soft flywheel key is "made" to shear to prevent major damage to the engine in the event of striking something solid.