Surging

90% of the time, to me surging usually means a lean condition? I assume it's at a steady/cruise rpm? The fact that you changed nothing and it just appeared, seams like it could very well be a bad tank of fuel.....Moisture? Trash in the carb is a possibility, but if you're running a fairly new filter, it's not likely. There's always the chance it's ignition related, but at this point i'm not sure I'd look there just yet. Hopefully some others may have a idea what's going on?