Hi,
I don't have any other flightsim addons on the 2nd machine. I do think that I may have fixed the problem. After much wailing and gnashing of teeth, it appears our old friend AVG may be the culprit. I have it installed on both machines - when AVG was disabled on both I then started up EFB. Low and behold, it connected with the aircraft in fsx. I the reapplied AVG and, at time of writing this, it all seems ok. Hopefully this may helf others who are having issues.
Thanks for your reply and suggestions.
Steve Amsden