Good points, thanks.
I have just ordered a ds1307, so hopefully I'll patch that in next week. Overflow does make sense, even though (I think) my Arduino code is free of counters. Perhaps something could be hidden in a library? When I started work on my machine I did use '<>' as start and end markers, but I still had the stalling problem after a few hours. I just use vbcrlf as line terminators now, which seems to work just as well. The lines never get any longer than the example I posted above. I am away from my Arduino sketch today (my Girlfriend has the laptop on a weekend course) but will post when I can.
@MikMo
That is very interesting indeed. I do have a lot of textboxes in my VB app, though none of them get updated during the run of the machine. I just set my parameters and press 'go'. I was running this from the IDE in debugging mode, I wonder if running the bare executable would behave differently? Also, you are very lucky to be living in CPH. It's one of my favourite cities, been there twice this year.