Had problems too:
-charger might not be original, check http://forum.xda-developers.com/showthread.php?t=2274321. mine is original so moving on.
-phone's dock might be too loose, put a little bit of tape on top of usb cable small connector; worked like a charm for me, fixed random disconnects from pc.
-cable might be broken internally, open and re-solder (shielding is also important); this fixed slow charging for me too.