I have had no problems with my iPhone 3G until I upgraded to the 3.1 firmware. Lately I have been noticing it gets very warm and it is frozen. To get it up and running again, I need to hold down the 2 buttons until it "reboots". Is this a problem with the firmware or something unrelated (such as a problem with the phone itself)? How do I downgrade to the 3.0 version of the firmware to be on the safe side? Thanks.