2000 alero Radiator drain plug problem ???

2000 Alero, 3.4 v6. I recently changed the water pump. drained, flushed, and refilled the coolant. twice since then the Radiator drain plug will come loose when driving down the highway...... coolant every where, car over heats.

The first time it happened I thought maybe I didnt close it properly when i did the water pump.. now that it has happened again I am pretty sure there must be a problem with the plug, or the radiator itself. Any thoughts on this woud be apreciated !

Thanks