Thanks for the chain tips.

You are probably right about it being operator error, as it happens at the same place every time I ride: at the bottom of a hill and going into a really steep hill- you know, one of those that you fear you might fall off your bike if you get get stuck in a too high gear. I think that I must change aggressively from the big chain ring to the small one and it slips off. When I use granny gear anywhere else, it doesn;t happen.

To make things worse, I am still trying to get used to my new road shoes and pedals and have ENORMOUS difficulty getting clipped back in and going again going uphill.

Thanks,
Sue