iphone 6 after restart needs restore

Hello guys,

i bought an iphone 6 with 64gb.

My problem is:

If I restore the firmware or via the dfu mode, it is done without any errors (now the latest 9.2.1)

but when i reboot/restart my phone, everything its lost and it hangs either on apple logo or stucks on restore mode.

have you any idea? i restored it many times, donwloaded many times new firmware file. still the same

but when i use the phone without any restarts, it is not crashing and working stable until the battery drops or turn off or reboot.

the phone was dropped once time. there is no long screw damage, no water damage.

thanks for your help