Since the Froyo update, I've been having some USB file transfer issues. After the phone has been running for a while, I no longer get the "USB connected" message when connecting to a PC, though the phone does start charging. If I reboot the phone, it works fine again, and I can transfer files with no problems for a while. Not sure what exactly causes it to stop working. Any ideas what the issue might be, or the best way to figure out what's going on with it?