On the stock cases the deck, did your builder fill in the voids with devcon epoxy? If he didn't that's your problem more than likely. Yamaha cut's water jackets that matches the stock cylinders on the top of the cases about a 1/4 deep they have to be filled in with epoxy. The top of the cases must be flat completely to match the cylinders. Between the cylinders on the top of the deck on the stock cases theirs water jacket wright in the middle that looks like a football that has to be filled in also thats the important one. Especially if your transfer ports are open up in that area. Your gasket will not cover that football completely.Theirs alot work to be done with the cheetha cylinders thats why they came out with the cubs. I wouldn't run it any more till you found the problem.