Serial Monitor affecting output

I have two way wireless (nRF24L01) working properly as per ...

Serial.begin (9600)
both NL&CR at 9600 on monitor. Only the nano (receiver) is connected to monitor

Hope the point by point is helpful enough.

Power both boards on and the system works
Start the serial monitor and it does not work
With Serial monitor open reset mega and it works
Close the serial monitor and it still works
Open the serial monitor It fails again
With Serial monitor open reset mega and it works

I have never come across this before and cannot find an answer on the open net. Hope someone has an idea for me
Thanks

You need to post the pair of programs that you are using.

...R
Simple nRF24L01+ Tutorial