Maybe the deck on the intake is warped and has failed under the gasket on cylinder 5. At some point 5 may be weeping into 6. I bet if you pull the intake there will be some signature left behind from the coolant. If it’s coolant that’s finding it’s way in there I’m sure you may have some misfiring or worse which is what your experiencing. It all seems like a wired problem. Almost like floating valves from week spring but I doubt that’s your problem. Good luck.