Thanks for the responses that others have the same problem.  I hope no one misunderstands, I think the CS 757 is a great add-on that has been made even better by 4.3.  However, in Version 4.2 the Lat/Lon did change as the flight progressed because I used it frequently in that mode selection, so there does seem to be something in 4.3 that is causing the problem.  And as several people commented, this is a very small problem given that using Shift+Z I can get continuous present position readout.  

I have to admit I was slightly annoyed when CS Support rejected my official report of the problem.  I fully understand that it's my responsibility to read and understand the manuals, knowledge base data and forum postings before going to them for an answer, but I had done all that, was managing the IRS by the book, and still had the problem.

Well, enough editorializing.  I say again, this is a great add-on (I also have the CS 707, C130, and 727; they are also outstanding, and I would recommend them to any simmer.