boost::io_service run() never returns sometimes - boost

Question

I have the following code to read data from a RS232 port. For some (from beginning) runs, io.run() returns immediately and m_handler() is called which is expected. but sometimes (a fresh run from beginning), io.run() goes into a inside forever loop(stacked inside forever) even for the first time called inside the while loop. And this happens randomly. What's is/are the potential problem(s) at here?
#define BUF_SIZE 512
char data[BUF_SIZE];
void my_handler(const boost::system::error_code& err, std::size_t bytes_transferred)
{
printf("%d\n", bytes_transferred);
if (!err){
process_data();
}
}
void test_serial_port( int argc, char* argv[] )
{
// hard coded parameters
const char *PORT = "COM1";
serial_port_base::baud_rate BAUD(9600);
serial_port_base::character_size CSIZE( 8 );
serial_port_base::flow_control FLOW( serial_port_base::flow_control::software );
serial_port_base::parity PARITY( serial_port_base::parity::none );
serial_port_base::stop_bits STOP( serial_port_base::stop_bits::one );
io_service io;
serial_port port( io, PORT );
// go through and set all the options as we need them
// all of them are listed, but the default values work for most cases
port.set_option( BAUD );
port.set_option( CSIZE );
port.set_option( FLOW );
port.set_option( PARITY );
port.set_option( STOP );
while( !kbhit() )
{
io.reset();
port.async_read_some(
boost::asio::buffer(data, BUF_SIZE),
boost::bind(&my_handler, boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred )
);
io.run();
Sleep(300);
}
}