TM4C1231E6PM: UART losing data

Dear Sirs

I'm using the UART5 to communicate with a ESP8266 module.

This is the configuration

ROM_SysCtlClockSet(SYSCTL_SYSDIV_4 | SYSCTL_USE_PLL | SYSCTL_OSC_INT);

ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_UART5);
ROM_GPIOPinConfigure(GPIO_PE4_U5RX );
ROM_GPIOPinConfigure(GPIO_PE5_U5TX);
ROM_GPIOPinTypeUART(GPIO_PORTE_BASE,GPIO_PIN_4 | GPIO_PIN_5 );
ROM_UARTConfigSetExpClk(UART5_BASE, ROM_SysCtlClockGet(), 9600,
                       (UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE |
                        UART_CONFIG_PAR_NONE));

I'm pooling the serial port instead of using the interrupt. This is the function 

int recvString(char *target, char *data, int timeout, bool check)
{
	int i=0;
	char a;
	unsigned long start = millis();


    while (millis() - start < timeout)
    {
    	while(ROM_UARTCharsAvail(UART5))
    	{
              a = ROM_UARTCharGet(UART5);
              if(a == '\0')
                  continue;
              data[i]= a;
              i++;
    	}
    }
    	if(check)
    	{
    		if (SearchIndexOf(data, target) != -1)
    		{
    		    return 1; //break;
    		}
    	}


    return 0;
}

My problem is that the I'm missing some received data. The missing data is at the beginning of the transmission ,after some bytes are successfully read from  the buffer . The error is aleatory. Doesn't occurs always.