Well, that is what I thought was true about FSUIPC - that there could be no way in which it could be related. It was the last thing I tried... and that undoubtedly was the culprit.
Version 3.75 is working ok for me, though. So also is version 3.999w. Perhaps there was some bug that was fixed after 3.998g that restored normal functionality? (It was 3.998g that was causing the interference.)
Anyway - all is good now. I posted here as I was unaware given the info on the schiratti site that FSUIPC has been updated so frequently - and using the latest version posted on this forum, the problem was resolved. The version on the website, though, is still 3.998g - which was causing the problem.