@mlangeI don't know if your engine has knock sensors but my 5.7 pcm 2009 nautique 196 does and after going thru pretty much what you have I discovered that I needed a new knock sensor and that cured the surge problem' I have 2 knock sensors one on each side of the engine block. You have to remove them to drain the engine block. You have to make sure that the single wire lead going into each one is plugged in properly. Hope this helps.