Does your USB boot successfully? If so, copy the EFI dir from the USB and paste it into the EFI partition of your HDD/SSD. That's what I had to do in order to finally get mine booting properly.
To view the EFI dir of the HDD/SSD, you need to run a couple of commands in Terminal. In terminal, type "diskutil list" to find the proper identifier for the EFI partition, and then "sudo diskutil mount disk1s1" to mount that partition - your identifier may be different. Mine was disk1s1. Once the EFI partition is mounted, just paste the EFI dir from the USB into the EFI dir of the HDD/SSD and reboot. There are a few extra things that are unneeded but it should work. Mine has been stable for months and successfully updated the OS from the App Store.