Thanks Michael,

You said: "If you use a 34.38 KHz", I meant 34 to 38 kHz interpreting the data sheet which defines a bandwith of the receiver "delta f 50% is 3 to 6kHz".
For the PC interface I actually used a 4060 CMOS chip with a 2.45MHz resonator giving a little more than 38kHz at 1/64 Output. The receiving signal has some phase shifts but it works with 1200.
After doing some further tests, I have to admit, that I cheated myself because I did a loopback test on the circuit. I guess the current of the IR transmitter caused the receiver to trigger somehow. So 1200 is ok, but with 2400 I have problems due to an interesting phenomen: the output of the receiver goes low and then sometimes goes high after 150usec and stays for 100usec there and then low again. This looks like a pumping effect of the internal AGC circuitry. These wrong pulses have always the same timing.. Strange. It does occur more regularly if the transmitter is closer. I might test other receivers. Which did you succesfully test? Thanks for the hint for using 38kHz devices (I would have chosen these too, but they were not in stock).

Werner