When using FT232RL FTDI, I always get 'Invalid head of a packet' error at about 47%. Various flashing programs such as FLASH_DOWNLOAD_TOOLS_v1.2_150512, nodemcu-flasher-master or esp8266_flasher were used, but with the same outcome.
When I try to flash other firmware, I usually end at 7% or so.
I also tried using Arduino Uno or Mega as serial to usb converter (grounding RST pin and connecting RX-RX, TX-TX) but in this case, I was not able to flash any percentage at all (although AT commands were working).
I have no clue, please help.