Thanks for your comments. In my case I don't think that was the problem as I had enabled those comms. My application would run for minutes to hours before this...

It was a while ago but if I recall correctly I discovered what was causing the problem when I looked at the Event Viewer and found that Nortons was causing...

I think I can close this out as I suspect I have found the answer. Norton's antivirus seems to randomly conclude that the comms between client and server via TCP/IP...