Working on the fact that the problem recurs every 60 seconds, I found a reference to "Vista Lag Spikes" and it seems that gamers have an issue with this, and there's lots of stuff online about it, apparently a Vista "bug" (although not sure why Gill's laptop doesn't have the same problem.....)
At the moment, I have downloaded a program called "Vista Anti Lag". This has allowed me to play songs over 7 minutes long without a break, and I even managed 13 minutes in one go! However, it will run for a (once again flexible) amount of time, and then iTunes stops. At this point, clicking on "play" again gives a "connecting to URL" message and then the connection to the Internet is lost. This in turn is not resolved by a simple reconnect, but rather takes about 5 mins to re-establish a connection
So, I am in the area of a solution, but currently have another problem (the lack of connection to the 'net followed by a long time before reconnection)
If anyone out there has come across this and knows the answer, I'd be eternally grateful!