Try taking a flashlight and pointing it towards the bottom of the dock connector. Underneath you should see a small hole with either a white or a red little swatch. If you don't see anything right away try carefully removing any debris or holding the phone at an angle. Next check the inside of the headphone jack on the top of the phone opposite the sleep/wake button. Again, you're looking for a white or red swatch of what looks like glossy paper on the very bottom of the phone. If you see red, your phone may have come in contact with some liquid and you may need to replace some parts. In my experience, this is most common indicator of the symptom you're describing.