[willy]

primer ball will cause all you are experiencing Kev, when it does it next check and see if it is collapsed on itself at all indicating a vacum situation, in other words clogged or plugged up on tank side of it or is it soft and easy to squeze indicating that it is letting air in somewhere. On my new boat went through three of them to get a good one and after that all has been fine.
you do need a primer bulb as there are times when you can lose your prime.
you also should have a in line filter under the hood of the engine, I think where the line goes into the carbs. check it for the same reason.
I do not believe plugs would cause your problem, but have you pulled them and checked their condition, as in dry wet oily and sooted up one diffrent than the other three?
My next step after checking those things would be to run her on an auxilliary five gallon boat fuel tank. If you have no problems you know it is from engine back on fuel system, if you still have it you can still have a fuel delivery problem with the fuel pump needing a rebuild kit cheap and fairly easy to do your self or a clogged carb jet or sticking fuel bowl float.
If those things come up clean then you are into taking it to a tech who can diagnose the electrical system for a timing power pack coil problem.
On my merc it did something like this as the power pack started crapping out and then would not run at all.
I don't think you have a timing type problem though as it would run crappy most of the time if not all of it. Also do not discount the idea of the tank being dirty as they can come new with crap in them and get dirty during install or have a clogged or defective pick up in them.
I am afraid like me you are going to have to slueth this one out. But if it get to that point run it in and have a pro look at it before your season flys by. It is a low hour engine and a very good one so it is going to be something pretty basic I would think.