I have a split level house, circa 1958, with foundations not monolithic. Cracks above the doorway beside stairway to upstairs is cracking no matter how its patched. I was wondering- If I take out drywall and put up 1/2" AC plywood in one piece above doorway, screwing onto studs, except directly above doorway corners, finish plywood with drywall,- if that would keep the cracks from coming back. Another idea is to cut bottom wall plate loose from floor and let float on spikes like I've heard done where moving floors are a problem. Any tried and proven answers out there?