Curiously, the same thing happened to me about a week or so ago, causing heavy chunk lag until it stopped. I was able to track its "progress" by Forestry's apatite generation logging, but I couldn't determine if it was an entity or anything.Does anyone know what might have caused this pattern of chunk generation? The line goes right through 0,0.
... do you have Mystcraft installed? Could this be a result of the profiling?