This thread has become somewhat of a soliloquy, but no matter. It's still useful for me to keep a record of what I'm trying, and much neater than the ever-thickening stack of sticky notes accumulating on the edge of my machine.
Not sure I can quite blame VB/MS just yet. I did a rebuild of my VB app using .Net Framework 2.0 as there are many reports going 'round that 3.5 is bad news for serial comms. The machine still froze after that, so I added a Try/Catch around the serial sending sub in the VB. Okay, it froze again after about 11 hours, but something unexpected happened. I had coded the serial exception to throw out messagebox with the error, but there was no box when the machine froze!
May be the Arduino misbehaving after all, but just to be sure I have started another test job with a camera watching the Tx/Rx lights on the Arduino. Hopefully, when it freezes again I will be able to play back the footage and see if that little Tx flashed before the machine stalled.
Onward...