Werner.
At 2400 baud the 16X clock used by the receiver to strobe the serial data would be at 38.4 KHz. If you use a 34.38 KHz frequency to modulate the transmit buts it seems to me that you are subjecting the transmit bits to more than 1/16 of baud rate of error. (IE the 34.8 KHz cannot be synchronous with the the transmit UART bit edges). I have found that when I built just what you did, that it seemed to work better when the modulation clock was the 38.4KHz 16x clock itself and makinf sure that it is indeed sunchronous with the transmit data edges.

When doing this then you can use an IR detector that has a center frequency of 38 KHz as well. When you use a 36KHz detector module and then pass a 34.38 KHz signal through it you would find that the IR receiver detector/filter causes more edge distortion of its output than if the input signal is right on center frequency.

Michael Karas