I found this site yesterday, as I been having the same problem for over a year. My backround is an electronics design engineer in the powwer area. I have taken my Kindle apart several times, and thought the intermitten problem was the micro USB connector. But I thought it was the contacts that interface with the USB cable. After reading on here some of the post, I took it apart again this morning. This time I took the USB connector out of the frame and looked at it under a microscope. The interface connector pins were fine, but the pins that make conncetion the board above were really oxidized, especially the negative pin. In my opnion thats the real problem. It's a bad design from the get go.
I then wired brushed the pins, and bent them foward a bit, reassembled the unit. It started charging as soon as I pluged it in. This connector would have been ok to use if it were soldered to the board, but mold to the plastic case would had to have been changed to give it clearence to fit in, without distorting it when assembled.
Larry E.