Flash ESP8266 using arduino

Hello, I have hooked up a ESP8266 to an Arduino nano using the SoftwareSerial library (pin 10/11). I would like to Flash new code towards the ESP using the same serial connection. (I don’t want to remove the ESP for flashing new firmware).
If I run the SoftwareSerialExample sketch @57600bd I can easly transfer quite large amounts of data without an loss from my PC (connected to the the arduino USB) towards the ESP8266. Sofar no problems.
When I want to flash the ESP I place a 10uF capacitor over the arduino reset and groud (to prevent its reset during the upload), pull GPIO0 to ground, toggle reset on ESP and start upload using arduino IDE with proper board settings.
This initially seems to work but does end in a failure.
The message I get is:

esptool.py v2.8
Serial port COM3
Chip is ESP8266EX
Features: WiFi
Crystal is 26MHz
MAC: 18:fe:34:f2:xx:xx
Uploading stub...
  Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.7.4/tools/upload.py", line 65, in <module>
  File "C:/Users/Administrator/AppData/Local/Arduino15/packages/esp8266/hardware/esp8266/2.7.4/tools/esptool\esptool.py", line 379, in check_command
    raise FatalError.WithResult('Failed to %s' % op_description, status_bytes)
esptool.FatalError: Failed to write to target RAM (result was 0107)

My question is: what am I missing or does the ESP need stricter timing and is read-from-one, write-to-other serial line (ViseVersa) just not fast enough?