AVR USART Comportamiento aleatorio

Parece que estoy obteniendo caracteres aleatorios en mi salida de USART con un ATMEGA8

uart.c

#define FOSC 8000000
#define BAUD_RATE 9600
#define UBRR_VALUE (uint16_t)(FOSC/16/BAUD_RATE-1)

#include "uart.h"

volatile u8buf UART_RCV_BUFFER;
FILE uart0_str = FDEV_SETUP_STREAM(UARTSendByte, UARTReceiveByte, _FDEV_SETUP_RW);

ISR(USART_RXC_vect)
{
    //got a byte from USART. add to buffer
    volatile uint8_t data;
    scanf("%c",&data);
    if((UART_BUFFER_WRITE(&UART_RCV_BUFFER, data)==1) || (data=='.'))
    {
        //either buffer full or '.' received
        //stop the UART receiver unit + disable interrupt
        UCSRB &= ~((1<<RXEN) | (1<<RXCIE));
    }
}

ISR(USART_UDRE_vect)
{
    volatile uint8_t data;
    if(UART_BUFFER_READ(&UART_RCV_BUFFER, &data)==1)
    {
        //end of buffer reached. nothing more to transmit
        //disable RX
        UCSRB &= ~((1<<RXEN) | (1<<RXCIE));
        //reset buffer
        UART_BUFFER_INIT(&UART_RCV_BUFFER);
        //enable  RX side
        UCSRB |= ((1<<RXEN) | (1<<RXCIE));
    }
    else printf("%c",&data);
}

void UART_WRITE_STRING(char* str)
{
    //Sending a string without any command
    //disable RX/TX interrupts
    UCSRB &= ~((1<<RXCIE) | (1<<UDRIE));
    printf("%s",str);
    //re-enable RX/TX interrupts
    UCSRB |= (1<<RXCIE) | (1<<UDRIE);
}


void UART_INIT(void)
{
    UBRRH = (uint8_t)(UBRR_VALUE>>8); //set UBRR register
    UBRRL = (uint8_t)UBRR_VALUE;
    UCSRC = (1<<URSEL) | (1<<UCSZ1) | (1<<UCSZ0);//set frame format: asynchronous,8data,1stop,no parity
    UCSRB = (1<<TXEN) | (1<<RXEN) | (1<<RXCIE) | (1<<UDRIE);

    UART_BUFFER_INIT(&UART_RCV_BUFFER);
    stdin=&uart0_str;
    stdout=&uart0_str;  
}


void UART_BUFFER_INIT(volatile u8buf *buf)
{
    buf->index = 0;
}

uint8_t UART_BUFFER_WRITE(volatile u8buf *buf, volatile uint8_t data)
{
    if(buf->index<UART_RCV_BUF_SIZE)
    {
        buf->index = data;
        buf->index++;
        return 0;
    }
    else return 1;

}

uint8_t UART_BUFFER_READ(volatile u8buf *buf, volatile uint8_t *data)
{
    if(buf->index>0)
    {
        buf->index--;
        *data = buf->buffer[buf->index];
        return 0;
    }
    else return 1;
}

int UARTSendByte(char u8data, FILE *stream)
{
    if(u8data=='\n')
    {
        UARTSendByte('\r',0);
    }
    while(!(UCSRA&(1<<UDRE))){};
    UDR = u8data;
    return 0;
}

int UARTReceiveByte(FILE *stream)
{
    uint8_t data;
    // Wait for byte to be received
    while(!(UCSRA&(1<<RXC))){};
    data=UDR;
    //echo input data
    UARTSendByte(data,stream);
    // Return received data
    return data;
}

main.c

#include "uart.h"
#include "twi.h"



int main (void)
{
    DDRC = 0x01; //set PC0 as output
    PORTC = 0x01; //turn on PC0
    //PD0 = RXD = INPUT
    //PD1 = TXD = OUTPUT
    DDRD = 0x03;

    UART_INIT(); //initialize UART
    sei(); //Enable global interrpts

    //_delay_ms(500);   

    UART_WRITE_STRING("Hello World ! 1:)\n");
    UART_WRITE_STRING("Hello World ! 2:)\n");
    UART_WRITE_STRING("Hello World ! 3:)\n");
    for(;;)
    {

    }
}

Esto en la ejecución debe darme

Hello World ! 1:)
Hello World ! 2:)
Hello World ! 3:)

Pero lo que obtengo es

\n\nHello World ! 1:)
Hello World ! 2:)
Hello World ! 3:)

el número de \ n es aleatorio. A veces no hay ninguno, a veces 1 o 2 o 3. ¿Alguna idea de lo que estoy haciendo mal aquí?