I think it's simple, the plugs are "loading up" and it takes a few cycles for them to clean out and fire proper again. It's running temporarily rich, could be due to hot fuel over boiling a bit from the carbs when you first take off with a hot engine thats been sitting OR turning a corner or in some cases just sitting at a long light with a lumpy idle.
I don't know of any good solution for this, check float level and set it as low as possible is all I can think of.
OK, not so simple, but thats my 2 cents.
