Because of this, I can't flash it. It turns on a few seconds after plugging it in, although it get recognizes that it's in flashmode the few seconds it remains off.
Can anyone help me?
If the USB connection is lost for short time, the phone exits the flashmode and when connection returns, phone boots up to charging mode.
That happens often with my Xperia Arc, thereby I hold down the button even after connecting phone in flashmode to avoid it.