[Al Skierkiewicz]

Mick,
This is the first time I have heard of this problem. There are some various issues with both towers that have been discussed at great length in a variety of places. I did a search to see if I could gather any additional info for you but didn't really find anything important. As the RCX is a S L O W processor and USB is a pretty fast transfer, I suspect that the port is just running too fast for the port on the RCX to keep up. It is shutting down because it can't see the handshake from the RCX in time to wait before transferring the next chunk of data. You might want to try using the supplied software (for the USB convertor) to slow data rates down to a speed the RCX can keep up with.
I am assuming you have already gone through all the problems that might be interferring with communications at the IR level. i.e. IR emmiters like cameras security illuminators, remote controls and other towers can all interfere with communications. Sometimes a simple light proof box over the tower and RCX during programming does the trick.