Unfortunately I had to finish my project quick, so I used the 0018 IDE to do that. Now I don't have any Duemilanove to examine the problem any further (the Duemilanove is at my client).
I still have a Mega (1280) and "hoped" I could reproduce the same error with that board, but the Mega works perfect, no error.
I try to get a new Duemilanove to examine the problem again.