Again you come to my aid Darrel.

I had limited the output by declaring the lat/lon secs as byte. Changed them to word and it becomes much more stable now. Datasheet says the decimal part is of variable length not fixed, so word should cover it.

I have also improved the start time to fix by finally fitting the Supercap to the gps module, something I should have done a while ago. This gives 48hr backup.

Don't groan but I now have another problem I'm going to look at. The time is now 2 seconds slow. Probably all the code plus the NMEA being slow too.
I'm looking at adding 2 secs then trigger display using DT_Ints on the next PPS pulse rise time. Feasible ??