Do you have a micro SD card installed? If so, remove it for a while and see if it makes a difference.
The freezing up doesn't concern me--that is it's something that can be fixed--and I imagine the back key not working is because the device is hanging. But if you are having button and/or keyboard malfunctions along with other strange behavior, that is something to worry about.
If you've exposed the device to moisture, even a humid shower room or sweaty pocket, you need to remove the battery NOW and see Got Your BlackBerry Wet? - BlackBerryFAQ
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