Arduino nano (clone) needs serial connection

Hi, I have got a strange bug.
I bought Arduino nano clone for less than $2 with the CH340 USB serial converter to see if it is even possible - I thought the ATMega alone costs more (it definitely does in our local store). Everything looks fine apart from one thing.
I need to have Serial Monitor opened for the Arduino to work - even if Serial is not used at all. If I close the Serial Monitor the Arduino stops doing whatever it does and start blinking the "L" LED regularly with about 1 or 2 Hz frequency. When I open the Serial Monitor again the sketch runs as expected (I know it restarts the Arduino) and after closing it the blinking starts again. There is my minimal example:

void setup() {
  pinMode(13, OUTPUT);
  digitalWrite(13,LOW);
  pinMode(8, OUTPUT);
}
void loop() {
  digitalWrite(8,HIGH);
  delay(1000);
  digitalWrite(8,LOW);
  delay(100);
}

With Serial Monitor open the LED connected to pin 8 is blinking while onboard LED is off. As soon as I close Serial Monitor the external LED turns off while the onboard LED is blinking (with duty cycle approx 5-10%).
Thanks for any advice!

EDIT: I tried measuring voltages on pins. In general voltages are the same as on my (working) UNO: stable 5V is on 5V output and TX and RX pins (digital 0 and 1), on pin 13 is alterning 0 and 5 (as the LED is blinking), all other I/O pins are floating.

Apparently there are problems with Arduino IDE 1.6.8. Try IDE version 1.6.5.

Thank you, it's really the Arduino IDE. When plugged into USB the Arduino is working as long as Arduino isn't runnig. When I start Arduino strange behavior happens (on board LED blinking) and when I close Arduino the external LED start blinking.
Does anyone know how is it possible? Will try it on older Arduino when I get time.

The reason it keeps blinking is due to a bug in how the 1.6.8 IDE looks at serial ports. They totally messed it up on windows, and the IDE polls the serial ports every couple of seconds in a way that makes the serial adapter act like it's opening the port. When a serial port is opened, it resets the arduino board (unless autoreset has been disabled) - and the bootloader which runs immediately after reset blinks the led.

Any workaround other than downgrading the IDE?

Go up to 1.6.9?