http://www.spacer.com/news/tectonics-02b.html
"Columbia began to break up about 1.5 billion years ago, and its fragments moved around the Earth independently for several hundred million years, he said. About a billion years ago, the fragments came together again to form a new supercontinent, called Rodinia. Rodinia lasted until about 700 million years ago before it too broke into several fragments.
These chunks moved independently until 250 million years ago, when the supercontinent Pangea formed, Rogers said. Pangea then began to break up almost immediately to form the world's present continents."

I've been sort of wondering lately whether, impactors instead of convection is the main factor in plate movement and the breaking up of supercontinents.
I realize I could get a fair amount flak for mentioning this idea:).
Or maybe the convection currents are responsible for bring continents together, then perhaps the impactors distrupt them. Otherwise why would convection bring continents together, then tear them apart- you would think it would do one or the other and not both. And if it did both it would seem less likely you would get a supercontinent. If above theory is right it seems that there is some process which makes supercontinent- oppose to a chance event.
-gb