I don't know enough about this to help anyone, but I found it fascinating.
Nice job guys.
I would appreciate your explaing something you said, John

"During normal operation, NMEA transmissions occur every 2 seconds.
For those who are familiar with software programming, the interesting
thing is that the error begins at 23:59:59 GMT and normal
transmissions resume at 00:08:34. The time interval of 8 minutes and
32 seconds just happens to be 512 seconds"

I know little about programming but understand that the number 512 is a
significant binary # and may be involved in some chance programming snafu.
However, I don't see how you got an elapsed time from 23:59:59 to 00:08:34
of 8 min and 32 seconds, i.e. 512 seconds. I get 1 sec elapsed until
00:00:00 then 8 min 34 secs for a total of 8 minutes and 35 seconds which
totals 480 plus 35 or 515 seconds, not 512.

Did one of us count wrong?

Thanks.
Chet