[356RS]

If I'm reading your post right, the leak is a pressure leak or compression leak between cylinder and head. You have tried 3 times to get the seating surface of the cylinders to heads better. Sounds like you might have a miss match of studs. After the heads and cylinders get up to temp the pressure leaks start. If the top studs are expanding, stretching, faster than the bottom studs, or visa-versa, you will have a compression leak. Assuming you had the machine work done correctly. What stud set are you using? With that much compression I would be using 993TT studs.