If you are sure that everything you tried has failed then your next step may be using the warranty, or insurance.
One of the users I support has a 8700 that won't charge.
I have tried new cables, new batteries and installed the new o/s with no luck. When it is plugged in it just keeps losing juice.
Does anyone have any suggestions?
Are the contacts clean that touch the battery? or bent?... hmmm if you tried all the options and nothing works, then i would do what CP said and take it back & see if your warranty or insurance can replace it if it isnt brand new....
Sportin a kBerry Curve