If that's the case then I'd lean toward a bad head gasket. I'd yank the head and put a straight edge to the block and make sure it's not warped. Did you clean the block til it was perfect before putting the new H/G on? Any debris or inperfections could have caused you to have an issue. And the oil galleys run through the head gasket so there could be something under the gasket letting coolant and oil into the combustion chamber. A good leak down test would tell you this for sure.