Leonardo (ATmega32u4) spontaneous reset on PORTF ?

Development Tools IDE 1.x
1

I am having issue with Arduino Leonardo board (ATmega32u4) resetting itself when I write series of pulses on PORTF (analog pins A0-A3), configured as digital outputs ?

The code illustrating the issue:

#define GOOD_VAL1 0x01
#define GOOD_VAL2 0x10

#define BAD_VAL1  0x5A
#define BAD_VAL2  0xA5

void setup() {
  Serial.begin(9600);
  DDRF = 0xFF;       

  // I also tried this:
  /*
  MCUCR = (1<<JTD); 
  _NOP();
  MCUCR = (1<<JTD); 
  */
  // Which compiles as:
  //   ldi     r24, 0x80
  //   out     0x35, r24     
  //   nop  
  //   out     0x35, r24     
  // Based on ATmega32u4 datasheet:
  // 26.5.1   MCU Control Register – MCUCR
  //   Bits 7 – JTD: JTAG Interface Disable
  //     When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. 
  //     If this bit is one, the JTAG interface is disabled. 
  //     In order to avoid unintentional disabling or enabling of the JTAG interface, 
  //     a timed sequence must be followed when changing this bit: 
  //     The application software must write this bit to the desired value twice within four cycles 
  //     to change its value. Note that this bit must not be altered when using the On-chip Debug system
  
}

void loop() {

  while (Serial.available()) Serial.read();
  Serial.println("Send something to begin...");
  while (!Serial.available());
  
  noInterrupts();
  for (char i=0; i<40; i++) {
    PORTF = GOOD_VAL1;
    _NOP();
    _NOP();
    _NOP();
    PORTF = GOOD_VAL2;
  }
  PORTF = 0x00;
  interrupts();
  Serial.println("Now you see me...");
  // Shows as series of 20 puleses @2MHz on logic analyzer

  while (Serial.available()) Serial.read();
  Serial.println("USE AT YOUR OWN RISK!");
  Serial.println("Send something to reboot the Leonardo...");  
  while (!Serial.available());

  PORTF = 0xFF;
  // The problem seems to be below  ?
  noInterrupts();
  for (char i=0; i<40; i++) {
    PORTF = BAD_VAL1;
    _NOP();
    _NOP();
    _NOP();
    PORTF = BAD_VAL2;
  }
  PORTF = 0x00;
  delayMicroseconds(16); 
  interrupts();

  Serial.println("Now you don't");
  // Actual result, LED is blinking rapidly for few seconds...
  delay(1000);    
}

There is noting connected to the board except the USB cable for power and serial communication. If I upload the sketch above and press Enter two times in the serial monitor, the board reboots. I also used logic analyzer and all I can see is the board all pins (A0-A3) go high 1us after last port output...

I would appreciate any help in reproducing the issue and possible causes for the reboot ? I am new to the forum so please accept my apologies if I am not following rules of the house.

Thanks,
Ivan