What is causing my butter toffee to separate? Each year at this time, I make 4-6 batches of Butter Toffee (2c sugar, 2 c butter, 2 tsp vanilla, 6 tbsp water), but only about half usually turn out ok. About half way through the heating process, the unsuccessful batch starts to separate. Once, I saved it by doing something magical to the temperature and stirring vigorously, but I don't know what I did. Every other time, Once it starts separating, it is a lost cause. 

I use the same pan, same stove, same wooden spoon to stir, and I think I am either using heat that is too high, or too low and too long. Help!
If you are using an electric stove you might have heat fluctuations enough to make it separate.