Restore loop (being prompted to restore again after a restore successfully completes)
Troubleshoot your USB connection. If the issue persists, out-of-date or incorrectly configured third-party security software may be causing this issue. Please follow Troubleshooting security software issues. .
Next try placing in DFU mode and then restoring.
Try another cable
Then try restoring on another computer.
Last, make an appointment at the Genius Bar of an Apple store.