After a good deal of Googling I've come across a couple of suggestions to try: (1) If I have 'auto pause' on, which I do, start moving just before pressing 'start'; it seems that if pressing 'start' standing still, auto pause kicks in. Poor Garmin - it's only doing what it's been told to do! I doubt this is the cause for me as the Garmin stops/starts recording when well into run .... OR (2) try turning off 'auto pause' which apparently works for some. I've decided on the latter suggestion and will see what happens during tomorrow's easy run. In the meantime I'll do some further searching.
I have doubts that this will get rid of this recent problem but worth a try ... before Sydney Harbour 10km on Sunday!
About 'auto pause' - it's possible, with auto pause on, that the Garmin thinks I've stopped running when it's because I'm running so slowly it believes I've stopped!!