i am having a major issue with my galaxy s2. As of today it started bringing up the pass code screen by itself and freezing the screen with it on. the only way for me fix the problem has been to take out the battery. this whole ordeal was draining my battery very quickly so i decided to turn my phone off completely since i was at work. Then i come to find out that when i shut my phone down it immediately turns itself back on. once i got it connected to my computer charger at home it doesn't keep a connection, it goes in and out, i use a computer charger for it and my computer was telling me over and over again that it was being connected to a device then disconnected, this was happening rather quickly. also it would bring up the vlingo based driver mode by itself while it was sitting untouched.
I agree try a factory reset and if that doesn't work which honestly I don't believe it will call AT&T Customer Support and they will send you a replacement . One way or another it's definitely still under warranty, if you've had it for less then a month your local AT&T Store will replace it and if you've had it for more then a month call AT&T Customer Support and they will send you a replacment.