Yeah, when I do 10 gallon batches (ie, starting with about 12 or 13 gallons of wort), it looks kind of "spurty" like in the video I posted. It oscillates back and forth every few seconds, where you'll get a big "glurp" then it will mellow out for a few seconds, then another "glurp", etc. I don't see that in 5 gallon batches. My guess is its due to kettle geometry and need for higher heat in a bigger batch. When the height is taller than the diameter, the steam forming on the bottom tends to bunch up more and then when enough concentrates together, it comes up as a bunch. When I'm boiling 5 gallon batches (ie, closer to 7 gallons of wort), the height is less than the diameter (same keggle), so that bunching up effect doesn't happen anywhere near as much, and the effect is much more gentle.