So, I have discovered that there might be an issue with this setup. The stereo seems to live a life of its own from time to time. It changes source i.e. from TV to Radio(or similar) on its own. First I thought that it was due to plasma interference from my Panasonic TV. However, I have proven that the TV is not to blame.
I think that the problem is due to the fact that the datalink signal going into the Beolink converter is not connected and hence the level might actually change a bit from time to time. I think that this might cause the converter to send faulty instructions to the stereo.
So, does anyone have any ideas on how to avoid this? I don't know enough about the protocol to come up with a good solution but I would suspect that somehow forcing the datalink pin high or low might do the trick. Right now it's just floating around... :-)