Susanne and Gillian have probably hit thenail on the head.
I would simply add that parasites can cause this problem. Is the cat regularly wormed? A Bacterial problem e.g salmonella, campylobacter and various forms of E.Coli could also be a problem.
However, from what you say and bearing in mind the problem is inconsistent, I would doubt if it was a viral infection or bacterial infection but a vet is the only person properly placed to make that judgment.
Diarrhoea can be caused by many other non infectious things. As the others have said - stress is a possibility. Does Dorothy share a home with other animals? Do they get on? Have other animals come into the home through any cat flap? Is the cat scared of any members of the family &/or is she a naturally nervous cat with an unknown prior history?
For non-infectious diarrhoea problems:
1. A change of diet not gradually introduced;
2. Milk intolerance is sadly too common. Cow's milk is not normally tolerated by cats and totally inadvisable hence special cat's milk produced by Felix and Whiskers.
I would suggest that the owners keep a proper record of what Dorothy is eating and when. Can they identify if she is stealing or getting food elsewhere? Is she a hunter, asssuming she is an outdoor cat? What does she catch? They need to know food iher food intake and compare it with the occurrence of diarrhoea to see if there is any consistent correlation with certain foods, etc.
