To follow up, I have tried 12.4(20)T3, 12.4(24)T, 12.4(24)T1, all of them have the same symptom. I have downgraded back to 12.4(15)T9 and the network is stable again. I need at least 12.4(20)T because we want to implement IOS content filtering. TAC case is pending. I will post again when the situation is resolved.