and then plug in the AC adapter? If it boots normally, I'd say the sudden surge of current as the charger/adapter is connected is causing some enough disruption to lock the system. I'd also what sequence you are using to apply power. Do you connect the laptop to the charger first and the charge to the outlet first or do the opposite? It might make a difference but I'd tend to think it's best to plug these things in with power off and then bring them up.