4 ms Glitch

Dear all

I recently start programming STM32F103VB Arm based MCU, writing simple blinking led on my own HW for testing. it was OK, but i feel something goes wrong, for the reason writing so simple codes to test the outputs of mcu as follows:

#define ch1_led 8

void set_ports(void)
{
  GPIO_PortClock(GPIOA,ENABLE);
  GPIO_PortClock(GPIOB,ENABLE);
  GPIO_PortClock(GPIOC,ENABLE);
  GPIO_PortClock(GPIOE,ENABLE);

  GPIOA->CRH &= ~(0xFul);        // Set Port A.8 as USB_Mode Select
  GPIOA->CRH |= (0x1ul);     // Output 10 Mhz PushPull

  Delay(1);
  GPIOB->CRH &= ~(0xFFul<<20);     // Set Port B.13 as Sck and B13 as MISO
  GPIOB->CRH |= (0x83ul<<20);  // Set Otput 50 Mhz PushPull for port B13
                                // Input for B14
  GPIOB->ODR |= (1ul<<14);     // Set Pullup input for Port B.14
  GPIOB->BSRR |= (1ul<<29);    // Set Sck to high

  GPIOC->CRL &= (0xFFul<<16);      // Set Port C.0/C.1/C.2/C.3 as Channels LED and
                                // C.6/C.7 as A2D C.S. and Fo Mode
  GPIOC->CRL |= (0x11001111ul);      // Output 10 Mhz PushPull
  GPIOC->BSRR = (0x11ul<<6);   // Set C.6 to disable A2D (CS Active Low) C.7
                                // to High Means Fo=50Hz

  GPIOC->CRH &= ~(0xFFul);       // Set Port C.8 as A2d Gain and C.9 as USB LED
  GPIOC->CRH |= (0x11ul);    // Output 10 Mhz PushPull
  GPIOE->CRH &= ~(0xFFFFul);     // Set Port E.8/E.9/E.10/E.11 as MUX Selector
  GPIOE->CRH |= (0x1111ul);  // Output 10 Mhz PushPull
}

void led_on(int ch)
{
  unsigned long led;
  led=ch;
  GPIOC->BSRR=led;
}

int main(void)
{
  SystemInit();
  SystemCoreClockSetHSI();
  SystemCoreClockUpdate();
  set_ports();
  led_on(ch1_led);
  while (1);
}

it should turn on the led for ever but a 4 ms glitch on output with period of about 425 ms makes the led blink invisibly. does anyone could explain me why and how could prevent of such a glitches ?

best regards