One way you could test is to connect the serial to a port on a computer with a null adaptor and hyperterminal. Press the button several times with different variables: lights on, lights off(where ir to serial is), command by itself, command in macro, etc. Now that I think about it, is your command in a macro? If so, try giving a longer delay before and after the IR command that triggers the IR to serial.

As far as the buffer, I have never had an issue like that with panasonic.