What you are describing is surging, and like Bob, I am not familiar with
Saito's doing that,(they tend to be pretty steady). I would go after the plug first to see if that settles it down. I break in Saito's on the stock plug, and switch to the F to fly,(they are expensive).
In all honesty, I am not aware of anyone that uses anything other then an F plug on Saito's. Also, I use a lot more oil then are are using,and that tends to make my Saito's rock steady.