In the film piece, he refers to low blood sugars (which you've had suggestions about) and low electrolytes. Would it be worth getting Pepper and Liath to both have a drink of the rehydration liquids (you know the powder sachets you make up with 500ml of warm water??) which contain the necessary electrolytes an hour before you walk/run them, and to take some with you as a preventative - make Pepper sit and drink for a few slurps during the walks a few times?
Let us know how you get on. I love this site!
Denise