Buffer Overrun indicates that the UART's input buffer was not read(emptied) when the next character was received. At 9600 baud a character shows up about every millisecond. Your system occasionally gets busy and doesn't get back to service the UART in time. Other options are to try at 4800 baud if that is an option or use xon/xoff or rts/cts handshaking/flow control.

James