ESP8266 NodeMCU Serial Junk Data arduino IDE Serial Communication

Hi, i got some problem in NodeMCU when using with arduino ide,

 

# Hardware
- NodeMcu v.1.0
- Printed circuit board 
- Sim800c mini GSM board

# Software
- Arduino IDE v.1.8.2
- esp8266  v.2.2.0 board manager from [https://github.com/esp8266/Arduino]
- Software Serial library.


I am sending AT command to sim800c, RX TX connection circuit is like this,

 

Used flash tools:

    Espressif ESP FLASH DOWNLOAD TOOL V2.4
    esptool.py


What happened:

    - Tested my own project (written with Arduino IDE)
    - Used Arduino OTA (device is in difficult accessible location)
    - OTA worked several times
    - OTA failed, device not connecting to WiFi anymore, even after power off/on cycle
    - Removed ESP module, try to reflash over serial port with FDDI cable
    - Still ESP module fail to boot

Found error codes:
On serial monitor (74880 baud) module shows always endless loop of

Fatal exception (0):
epc1=0x40211668, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000000, depc=0x00000000]



# Ref:

- got some ref. from this link : [http://bbs.espressif.com/viewtopic.php?t=1606] 
- Measure the voltage of ESP8266's pin VDD.
- Use hot air gun to remove the wifi Chip from the PCB board and measure the resisitence of each pin to the ground to check if the chip is broken.
- Power on the ESP8266 module, measure the voltage of ESP8266's 3V3 pin.

# Analysis:

- Did not get any fruitful results.
- Finally after some try and errors i got noticed that, when my GSM module was on and i am using NodeMCU there is no problem in any communication. but when GSM module is off i am getting this whole matter..

# Result:

- So as per previous reference i got idea about GPIO pin resistance, it must be not connected with ground.
- In my circuit R8 5.6K is connected with ground. it is giving a problem, when my GSM module is in switch on mode, internal pullup will take care about that, but when GSM is in switch off mode i am geeting lot's of junk in serial monitor..
- After removing R8 5.6K, NodeMCU is smoothly working..