My friends daytona had a leak in the head gasket that caused his problem. He just recently fixed it even though it did not resemble a head gasket problem. He only got water in the oil above 4,000 RPM.
You may have a cracked head if your positive that your gaskets are sealed. The other place to check would be the intake manifold to see that it is sealing properly. The water jacket gets pretty highly pressurized from the feed from the jet pump, so it will find any weak links in your water jackets, passages and gaskets.
Also, what kind of exhaust set up do you have?