When i cant get it to take jailbreak plug into power let it boot up then try again i got mines going this way also i have 2 micro usb but only one is good for the job.
I had the same problem exactly David but eventually managed to fix it using a separate USB cable. Where did you get the USB cables from? I know it is painful but I would suggest getting yet another cable and trying again. Make sure the cable you buy is not some cheap Chinese version. The one I used that worked came with an Android phone.
Damn was hoping that 3rd cable was going to be it. Where are you plugging the cable into? Directly into the USB port on the PC or are you using any kind of USB hub?