I've tried the code with a DS89c420 developement board today, and there the problem also occurs.
So it seems to me it is a software problem and
I looked through the code, but so far nothing strange
seen. I rewrote a little part and tried that piece
alone and that didn't seem to give any trouble.
I will put a bit of the "old" code into the new one
and see where it will go wrong....