a half centimeter play at the rim is beyond significant. QRs are simple beasts - assuming you haven't put in a longer axle or anything like that, once you lock it up, it doesn't move about. It sounds like your bearings are loose. If you take the wheel off the bike, remove the QR and give the axle a wiggle, if you really have 1/2 cm play at the rim, you'll definitely be able to feel the axle move. That'll confirm that it's your bearings